"They were poorly catechized" means that when people leave the Church, it is not because of any flaw in the Church or its doctrines, but rather that they weren't taught the wonderful truths of Catholicism properly. 

To misquote Sir William Gilbert, "The Church is the true embodiment/Of everything that's excellent;/It has no sort of fault or flaw". Except, of course for things like rampant homophobia and sexism, hierarchs who cover up the crimes of priests and a few niggling things like that.