I was sick of working with this guy halfway through his blog post. To me it was pretty clear what they wanted, it was a "show us what you can do" kind of assignment while he wanted handholding and constant validation, already making excuses for being late and avoiding IMAP/POP due to "complexity" when applying for a backend email engineer position.

Yes, they should've told you not to bother after you sent that proposal. But they were completely right to ultimately reject you. You're a lot of work.