I think this is very insightful, and likely contains quite a lot of truth. I would like to suggest a second explanation, though, that may be equally (or more) important.

It starts with the observation--let's call it my "lived experience", shall we?--that the vindictive attacks are not made only by (supposed) allies of a group, but also by members of the group itself.

It continues by observing that off-target attacks by either allies or group members have a particular character, aimed at an (imagined) opponent that they frequently clash with. (Because I tend to call out people for bad argumentation regardless of whether I am sympathetic to their position or not, I get hit with this a lot.)

This suggests to me that a big component of the response is maintaining the purity of the tribal identity of the group. Of course, allies have to do it the hardest because they're in greatest danger of being rejected from the group, but nonetheless if everyone agrees that, I don't know, standardized testing is racist, then questioning that challenges the sanctity of the group and must be squashed.

Scott Atran, in In Gods We Trust (Oxford University Press, 2002), makes a reasonably compelling case that it is precisely the non-obvious statements--matters of faith--that most keenly distinguish group boundaries. These statements are, to a rational thinker, most deserving of question, since they seem central but do not seem to have adequate support. And yet, because these are covertly functioning as group-membership tokens as well as beliefs about reality, questioning these are particularly likely to result in your body being dumped into a ditch.

So I don't think it's necessary for the allies to be intending to keep oppressed those people who they claim to help. I think many of them honestly believe they've attached themselves to a noble cause and are doing their best to achieve justice. And yet, in order to show their loyalty to their cause, they, and the core members of the group, are driven to attack heretics. If they just let their intuitions run things and don't stop to think, that is.

Now, I think both things can be going on at the same time: some allies could be enjoying their superiority, while others are showing their colors...and some could be doing both.

But I think there's a compelling alternative for the same behavior that comes from different motives than you ascribe.