> The UI will be kept simple, showing pagination for sent and received emails. In addition to the requirements of the assessment, there will be a login screen ...
How would you reply to that?
One option is to tell them that the document looks fine, but also add that it does not actually describe the parts they are going to grade on (such as: "which part of aerc/mutt is does author imitate?"). But this is a bad idea, because the likely reaction of candidate is to spend even more time on spec... and the assignment is not about writing specs, the specs are not in the rubric, and you don't want to waste candidate's time on asking for docs you don't care about.
Another option is to tell them: "This spec is so bad, I don't think you can possibly pass. Bye-bye!". This is even worse, as it can potentially reject good candidates who are just bad at writing documents - and there are plenty of them.
So I think responding: "Looking forward to receiving your submission." is the pretty OK answer.
(I suppose the in the non-interview settings, if this was a junior engineer, I'd might also add something like "And don't forget to make the pages you write be terminal-inspired, as the requirements say, see videos of people using [aerc], [mutt] and [himalaya]" - but I can see why this was not said in the interview. After all, testing that the candidate can read and comprehend a tiny requirements doc is a part of the interview)