This doesn't make much sense, tbh.

The Pope is both a religious and (de facto) political leader, in much the same way the Queen of England is.

Being an atheist or a secularist (and I am unabashedly both) has nothing to do with anything here, unless you think we should discriminate in our funding of state visits on the basis of whether the leader of a state also happens to be a religious leader?

It seems to me that the Pope's visit doesn't constitute an endorsement or establishment of religion by the government who funds his visit.

This NSS point seems more libertarian than secular in its aim.

Not all secularists are libertarians, nor does being a secularist mean this sort of strange discrimination.

... That said... My personal opinion of any Pope is in line with this.