A polite response does not have to be a meek response. You can be blunt, you can point out all the problems, while still being polite.

That also has the advantage of being 100% clear.

"You just told hundreds of people my code is poor. You attacked me, and my professional reputation, and you did it inaccurately. This gives me little reason to respect you or want to help you. Asking without copying the world and without assuming you were flawless would have done a lot to help your case. Because I AM professional and you are new to the profession, I'll hold back on the reflex to point out to every one of these hundreds of peers that influence your career how rude, presumptive, and WRONG you were. Instead, I'm pointing out in private that you are wrong and not behaving in a way someone looking to succeed is behaving. This tolerance is not something you should expect from me again, and you should be grateful to be getting it now because your behavior does NOT call for cooperation. If you do this again, the next person will likely not show this patience and understanding. Do you understand that you made the error in your code? Do you understand how your behavior invites people to think poorly of you? Do you understand how to change both your code and behavior for the future? "

This has the senior developer takes on more work instead of a sense of vindication. However, this is much more likely to get a positive result in the long term - there's no mystery hope that someone that clearly missed out on social repercussions will suddenly understand them when attacked.

Being a senior developer is not about winning battles, it's about learning to win without battles, and even more importantly, it's about TEACHING that.

I've gotta say, if I was a third party to the exchange that involved your suggested message I would think the senior developer was a pompous and condescending ass. In the UK we (or at least I) wouldn't consider what you wrote to be a polite response.

Funnily enough, my last team worked with a software supplier from the UK, and I’m 99.99% sure the above text was copy-pasted from an email reply to one of my overzealous team members. We thought it was over the top, but chalked it down to British culture

There is also a difference between teaching and grandstanding; the reality of the situation is that there is no single correct response for this kind of situations: the junior maybe was zealous, maybe he was trying to shame the senior, maybe he just copied in CC the wrong mailing list.

Obviously the original answer was not appropriate in every situation, but also likely it was appropriate in some.