Could one do a "terminal inspired, mutt-like email client" that completely satisfies OP's proposal (web-based, TEMPL, pocketbase, pulumi, etc...)? Sure, it would be possible. None of those are _required_ for the submission but you can totally use them if you wanted to. So the interviewer probably thought, "hey this person is going all out", and was absolutely honest in saying "Looking forward to receiving your submission"

I cannot really blame the interviewer for not reminding the candidate that they should actually follow the requirements, in addition to all the optional stuff they mentioned in their spec.