These both actually seem like reasonable sentiments to me, but I don't think they are totally analogous.
The atheist, by rejecting fundamentalism, never explicitly claimed that they were also rejecting bigotry. The TERF explicitly claimed that they were rejecting anti trans bigotry (if their thing is, as implied by this post, "i'm trans inclusive now" and not, as seems to be weirdly common for ex-TERFs, "i'm a right wing tradwife now"). If the atheist failed to reject fundamentalist bigotry, that doesn't make them a failed atheist, just an asshole. If the ex-TERF claims to be trans-inclusive but remains transphobic, that makes them a failed ex-TERF.
But, for both, I also think it's reasonable to say "your beliefs seem to in practice line up with a set of beliefs that are prominent in at least one of your formative cultural spaces, and which contradict your stated values." For example, I think it's reasonable to point out that modern tumblr anti-sex panics look a lot like conservative homophobia, and point out that given that conservative homophobia is a big belief in American culture, people nominally against it but seeming to align with it in practice might be being influenced by it, especially if it was a big influence on their upbringing.
So I actually think both of these analogies are fine, although they don't quite line up.
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The atheist, by rejecting fundamentalism, never explicitly claimed that they were also rejecting bigotry. The TERF explicitly claimed that they were rejecting anti trans bigotry (if their thing is, as implied by this post, "i'm trans inclusive now" and not, as seems to be weirdly common for ex-TERFs, "i'm a right wing tradwife now"). If the atheist failed to reject fundamentalist bigotry, that doesn't make them a failed atheist, just an asshole. If the ex-TERF claims to be trans-inclusive but remains transphobic, that makes them a failed ex-TERF.
But, for both, I also think it's reasonable to say "your beliefs seem to in practice line up with a set of beliefs that are prominent in at least one of your formative cultural spaces, and which contradict your stated values." For example, I think it's reasonable to point out that modern tumblr anti-sex panics look a lot like conservative homophobia, and point out that given that conservative homophobia is a big belief in American culture, people nominally against it but seeming to align with it in practice might be being influenced by it, especially if it was a big influence on their upbringing.
So I actually think both of these analogies are fine, although they don't quite line up.