I can't answer your Q why that A was deleted. Those who deleted can explain that better. But after reading the answer as it existed with the rude word removed, no need to delete it as rude & abusive, says I. @Jarko Dubbeldam's editing out only the rude word and replacing with "way out of line" made the answer no longer rude or abusive.
Admittedly the answer is highly critical of OP's behavior. However, once the rude word was removed, the language of that criticism is neither rude nor abusive. Here is the full text of the answer with just the single edit. Make up your own mind:
You can apologize, but you can't make amends. You called out her weight in front of the whole class and there's no way to retract that. You shamed her and this will be on her mind for days to come and every time she steps into the classroom. She made a simple mistake and got called fat. Her being overweight has nothing to do with stepping on your foot. You were hurt and went for the easiest insult and hurt someone in a way she won't quickly shake off.
You can talk to her separately, tell her you had a preexisting injury on your foot and apologize for your remark, it was uncalled for. You can't excuse yourself out of this one, so don't blame it on being in pain. Being in pain at the moment is no excuse, because, again, her being overweight is not the cause for stepping on your foot, so you had no right to bring it up.
You were way out of line saying that, but you are showing remorse now and can learn from it. These are the mistakes we make and learn from. Next time you know not to go for the easiest insult, because you know a quick remark like that can have a disproportionately devastating effect.
I am not aware that criticizing the OP in language that's not rude or abusive goes against the 'be nice' policy. Or else we could never criticize OP because the post might get deleted!
I also think the last paragraph at least is constructive criticism. Maybe not a very good answer for OP's question? If so, downvote, downvote, no need to delete.
You probably agree as well, @Vylix, since you did ask this meta Q and noted that
"I don't think this answer is worth deleting. The facts in the question are reiterated in a blunt way, not rude (...) This answer gives a different perspective from the offended person's view, and help us to think how to think from the offended person's view (...) If you don't agree how the answer is delivered, please do not easily delete, but downvote instead."
I generally agree with OP, if not specifically about this particular answer. That does introduce the question, "when should a post be deleted as rude and abusive?"
May I make use of the valuable Meta.SE post kindly linked by @Tinkeringbell in a comment, this part of which answer is most relevant here:
A post should be marked as rude or abusive (...) if it contains hate speech, obscenities, abuse against people, or abuse of the community or system, i.e., a clear violation of the be-nice policy (...) As a rule of thumb, everything that would be out of place in polite discourse is rude or abusive. If an otherwise valid post contains vulgar words (...) edit the bad part out instead of flagging the entire post as rude or abusive."
The only thing that is debatable here is whether the answer that is at the centre of this issue had one removable rude word or whether the whole post is rude and abusive.
There is at least one previous example of an answer where the only one rude & abusive word alone was edited out and the answer was not deleted:
Rude/abusive flag declined?
4 notable comments from that meta discussion under @Catija's answer are extracted here:
But given the full context of being generally a terrible answer and being rude, shouldn't it have been deleted? – apaul Oct 19 [read full comment link]
We don't delete terrible answers, we downvote them. – Catija♦ Oct 19 [comment link]
Excellent answer and excellent decision (to edit and not to delete) @Catija. – English Student Oct 19 [comment link]
I'd like to add that I would like to see more mention of 'rude/abusive/bad' answers in chat (...) If it's a single word, we should definitely edit. – Tinkeringbell Oct 24 [read full comment link]
Note that by 'excellent answer' I mean @Catija's excellent meta explanation of the action taken in that particular case. She also reminded us that those users who have the right to edit posts can remove the offensive word without waiting for moderator intervention.
But we do need to maintain consistency in deleting or not deleting members' posts. So if more members agree then the answer you referenced can possibly be undeleted.