The problem here is that the cases that I'm referring to are people saying that they're feminist without believing in or following core tenets of feminism. If I said, for instance, that I was a Christian, and that I believed in sacrificing animals in the temple for receiving forgiveness from sins, you would quite rightly say that I was not a Christian at all, because I didn't believe in or follow one of the primary tenets of Christianity (e.g., that Jesus dies for our sins, and that he fulfilled the law such that animal sacrifice was no longer necessary; I am not, for the record, Christian, nor do I believe in the idea of sin). Most feminists would argue that the primary tenet of feminism is tearing down gendered power structures; reinforcing gender stereotypes would therefore be not feminist.

Well no, I wasn't making a judgement of whether or not those people were "true feminists". I was making a judgement of their actions as being unaligned with the beliefs they claimed.

Humans do things that don't align with their core values all the time. It's called making a mistake or doing a bad thing. For example I'm staunchly anti-racism and try to check my actions. Doesn't mean I never do racist things. Nor does it mean if I do a racist thing, suddenly I'm no longer anti-racist forever and all time. Who tf knows where the line is, but probably it's around the point where you stop trying.