Agree that editing of the last / most recent reply can be the strongest use case. But it can be the same for an earlier one too, depending on the "quality" of the additional posts that were added after the one being edited.
Meaning whether those additional existing posts affect whether "continuing to edit this existing post" will still make sense, same as you would evaluate about any new content Post Review notifies you about. Versus similar to when Post Review notifying you of a substantial new post, you need to "switch tactics" and now post a new reply instead. Which will be inclusive of this latest "substantial" information, rather than continuing to edit the earlier post with what you were trying to clarify.
But even if there was only the one strongest use use, I'd still give this idea my vote simply for nudging towards additional consistency in the behavior. I sit there composing a Reply not having to worry about "I wonder if someone else will have already responded with something I should check before posting", because I know I'll see it before my post is allowed. But when editing a message, there is no such assurance. And if concerned, in that case you have to check manually instead. Even though "Edit is just another mode of the same posting.php Reply interface", and the behavior could be consistent.
Note I'm actually not opposed to Topic Review being shown in the edit case, either. Again, mainly consistency: Why should it be harder to quote additional existing posts when editing, as compared to the ease of making those additional quotes using Topic Review when replying.
But to the concern you raised, phpBB already handles that. Because even in "Reply" mode there is a limit to the number of posts Topic Review will allow to be displayed. It won't just run away and display all the posts, even in "Reply" mode. Same would be expected if Topic Review became enabled when editing a post.
But this idea isn't about trying to get Topic Review enabled during editing. Just Post Review is all we're voting for here. Topic Review will either be its own separate idea, or maybe a judgement call by whatever developer tries to bring more unity and consistently to the "Reply" versus "Edit" mode of posting.php when addressing "Post Review".