So what does he have in mind, particularly as he accepts this guy was an atheist before he stopped being an atheist? What has a catholic upbringing got to do with this chap becoming a catholic in later life and switching from atheism?
It has everything to do with it.
There is a world of difference between, on the one hand, a person brought up as a catholic, for whom catholicism is a major part of their upbringing, their culture and their heritage, who might later in life move away from that belief system for a while and then revert back to it. And on the other a person who wasn't brought up in a particular belief system (e.g. catholicism), indeed perhaps in a non religious manner and later becomes an adherent of a particular belief (e.g. catholicism) without any upbringing, heritage or cultural engagement with that belief as a child.
They are chalk and cheese - realistically in the latter case the person
converts to being a catholic, in the former they merely
revert to being an active catholic. Indeed in the mind of the catholic church there would be no
conversion in the former case, nor really
reversion, as they'd consider this person to have been a catholic continually from childhood, albeit one that wasn't active for a while.