I’ll analyse it part by part. There are three sentences in that mechanic.

“A Failed Experiment may change groups by posting a Story Post with the [Group] tag that specifies one and only one group the Failed Experiment wants to join (using the group’s index).”

You may do the post, by the which you join (with the “may - by” wording being used in other places too, which I hinge on). I think we both agree with that the scam would work, if that sentence solely existed by itself. So, up to here, all clear.

“Members of the target group can refuse the request by posting a comment containing the AGAINST icon.”

You CAN refuse the request. Note that the fact that you CAN refuse it doesn’t alter what the key sentence means. Being able to refuse something or not per se doesn’t alter that I can do it or not. You can refuse that I take away your hat, for example, but that doesn’t change that I can take it away or not per se. UNLESS this refusal has some extra formal baggage, which is has, which is to support the next sentence’s mechanic.

“If after two days the number of group members voting against is less than half the size of the group (not rounded) then the Failed Experiment who published the post is removed from their previous group and then added to the target group.”

This mechanic triggers after two days and having computed the votes. Note how the second and third sentence work in tandem - the first recognizes the AGAINST votes, the other tallies them to trigger a new effect.

There is nowhere to be found an explicit mention that these two last sentences somehow condition the first one.

Of course, it would be desirable that they did for this mechanic to work as intended, but this exploit precisely hinges on that what this was trying to do, was expressed in the wrong way.

So,

This is a close call, but I think the fact the paragraph ends by saying “removed from their previous group and then added to the target group” is close enough in meaning to “changing groups” for it to be a clarification of how the paragraph started. The paragraph is how you change groups.

I thought about it a bit more and I agree this scam is not valid. It is reasonably obvious that the whole paragraph is detailing one global mechanic, and specific sentences within that paragraph are not meant to be interpreted separately.

Been getting doubts about how untracked actions are done in the first place.