Grabbing at straws, perhaps?
Most likely the writer considered himself to be a Catholic before being formally received into the Church. Admittedly, this simple explanation would not fit your theory.
However, rather than suggest that your speculation based on an apparent anomoly is a proven fact and then using this “fact” to suggest that conversions to Catholicm are bogus, you might want to dig a bit deeper.
It is this guy's alleged conversion from Calvinism that reeks of fraud. Either he was never a Calvinist or he never understood Calvinism. I suspect both, and FWIW I am not a Calvinist (although I was the only freeper to score 100% Calvin on the Theology test). :-)
Additionally there are at least three different versions of this "First Vision" story out there that I could find.