Q:

Question protection should block low-rep users with existing answers on the thread

On Physics SE, I just came across a surprising case which looked like a user with 1 rep posting an answer a year after the question was protected. Upon some investigation, I found that they'd posted an answer a while back, before the question, and I'm quite sure that this is what enabled them to post that answer. Here's a screenshot of a section of the question timeline, with circles for your convenience:

Delburt Phend, the author of the posts (link to physics profile) probably never had more than 5 rep at any given point of time.
I think that they should have been blocked from answering that; both answers are poorly received because they pitch the same factually incorrect idea, and the second one includes a statement "Let's hope that the moderator does not read this".
Can the system be modified such that low-rep users cannot re-answer questions after protection? I only see possible cases of misuse, like this one, and I can't think of any circumstances under which this would be appreciated. After all, they'll still be able to edit their old posts and there's almost no chance they'll be aware of the guidelines for posting two answers to the same question.
If anyone's interested, I'm attaching pictures of the two answers. They aren't important enough to be pasted properly as text, so screenshots must suffice: https://i.stack.imgur.com/OaJwm.png (new answer), https://i.stack.imgur.com/gdWqZ.png (old answer). Here's the question on Physics SE

A:

The author of that question did have a reputation of 14 until a few hours ago. You can verify this rather easily via a SEDE query.
Earlier today, they lost 4 points to downvotes on a different answer, and then another 8 points when their previous answer to the question you cite was converted to a comment (this answer had 1 upvote and 1 downvote, for a net gain of +8 prior to its deletion). The remaining point was lost to the first downvote on their new answer.
14 points is sufficient to post an answer to a protected question, which they did. This action no doubt garnered them the very attention which resulted in the subsequent loss, leading us to the current situation.

