What I find interesting is that this person did not want to subscribe to the Westminster Confession, but rather chose a far more restrictive set of confessions in the form of the catechisms. Futher, the author does not go into which articles of the Westminster Confession they have a disagreement or find incorrect.
I'm not sure that's the way to look at it. To me it sounds like he's saying he thought there was a contradiction between claiming a "sola scriptura" standard AND the Westminster Confession as a another standard.
It's not that he found anything in particular wrong with the content of the Westminster Confession itself. It was that saying Bible ALONE PLUS Westminster Confession struck him as having a kind of self-incompatibility.
Maybe I'm projecting. It was something SORT of like that that persuaded me finally the leave the Episcopal Church. Particular teachings prompted me to look around, but it was the manner in which those teachings were arrived at, adopted, and discarded that persuaded me the Episcopalians simply could not be right.