Alexander’s piece begins its errors in its title:

We are not descended from Adam and Eve – but still, Jesus was born to save us

The Bible explicitly says it was to save us from the original sin of the aforementioned non-existent people – so no, the idea won’t work. But Alexander’s preoccupations lie elsewhere:

behind the question lies about 1,600 years or more of church history.

So, less than the age of the Bible then. Which means it’s not based on the Bible, but on efforts to make the Bible fit with later findings. That Alexander is guilty of this will becomes clear soon:

the Bible suggests otherwise. The tradition of interpreting the early chapters of Genesis figuratively goes back to two great thinkers from Alexandria

But that’s not the Bible suggesting it; that’s some of the Bible’s earliest apologists insisting the Bible doesn’t mean what it says. In other words, it’s the “thinkers” suggesting it, not the Bible!

Figurative understandings of the Genesis text have been part of mainstream theology ever since.

So have literal understandings of Genesis. And Christians have killed each other for centuries over whether this bit or that bit in doctrine is literal or not: just look at transubstantiation and consubstantiation. Meanwhile, a literal understanding of, say, the Resurrection has always been pretty much universal, and still is. But how does anyone tell which bits are which? By conceding the literal wrongness of as little as possible given the findings of science, history etc. I’m tired of theists using the fact that metaphors exist at all as a get out of jail free card for every single Biblical bit of nonsense. Are we supposed to metaphorically stone to death engaged rape victims and, if so, what does that metaphor mean?
Of course, you might argue this bit or that bit really is blatantly a metaphor because of what it says, like in this argument:

The first mention of Adam in the Bible is clearly referring to humankind (Genesis 1:26-27) and the definite article in front of Adam in chapters 2 and 3 – "the man" – suggests a representative man, because in Hebrew the definite article is not used for personal names, with Eve being the representative woman.

That’s just a guess: if there’s only one man in a story, he may be called “the man” whether or not he’s meant to stand for lots of men. But even if this argument worked, it would actually not be an argument for that section being metaphorical, but for its literal meaning being such and such. This is what’s wrong with theists using the “metaphor” trick: they’ve become so used to doing it without thought they might resort to it unnecessarily.

The Genesis narrative tells the story of humankind going their way rather than God's way.

It tells the story of Adam and Eve doing that. Were all women created from the ribs of all men? This “stands for us all” stuff is blatant nonsense. In any case, Alexander can’t even keep his story straight:

the New Testament does teach that humankind stays true to type – all people sin by their own free will – and Christ dies for the sins of all. Christ is the second Adam who opens up the way back to friendship with God through his sacrifice for sin on the cross. The result is the "at-one-ment" that the first Adam – Everyman – is unable to accomplish by his own efforts.

But that comparison means either Adam is only one guy like Jesus or Christ represents us all like Adam, and Alexander will never conceive either of those options. Also, this definition of sin contradicts the whole of the Old Testament, which Jesus said remains valid indefinitely – and even if Christianity doesn’t agree with that, it would mean Judaism – which follows the OT – has a falsity about it Christianity doesn’t concede in itself, which seems quite anti–Semitic.
After that, the mean number of errors per paragraph escalates for the conclusion:

Evolution's gift is a complex brain that endows humanity with free will, enabling personal moral responsibilities towards our neighbour and towards God. We are not puppets. God's gift at Christmas is forgiveness and new life through Christ for those who realise how far we've fallen from using that free will responsibly.

Going back to the evolution problem Alexander raised, where in the family tree of primates did Hell first await those who dissatisfied this still unproven god, and why – and how, given the gradualness of evolution? No primate species has first members. And why do we have responsibilities to God? Being omnipotent, He cannot be harmed. In any case, Christianity never concedes we should apologise to or otherwise make amends to those non–divine people we’ve wronged, so don’t pretend that Christianity recognises our responsibilities to our neighbours; the point of confession is that, even where we have harmed humans, it is God we need to appease. Also, “free will” is rather at odds with what we know of neurology – and again, if it’s applicable to us but not to, say, chimpanzees, how does that evolutionarily work? And as for this Christmas gift (shouldn’t it be Easter gift?) nonsense, guess what: humans haven’t really fallen, as per either a literal or figurative reading of Genesis; our ancestors were not more ethical than us. Look at the homicide rates per capita in primate species, and include humans in that analysis. Read Steven Pinker’s recent book on the decay of violence. We are not a fallen angel; we are a risen ape.