Did you miss the part where he said he was a Calvinist for 20 years?
Actually, if you read it closely, he claims he was a Calvinist for 21 years, not 20. But growing up in a church with 'Reformed' in its name, and even going to Calvin College doesn't make him a Calvinist. Indeed, as soon as he got out from under his parents direct supervision, he headed to a Baptist church although Grand Rapids has over 90 Reformed congregations, a handful of Presbyterian churches, and even at least Reformed Baptist group (not the Baptist church which the author attended.)
So his claims to have been a Calvinist from birth ring pretty hollow.