They're not asking for a disproportionate amount of time, the candidate just gave that amount of time, whilst still missing the brief.
the test asks for a very significant amount of work, and makes it clear that they are looking for more effort than the minimum ("Do the project in a way that shows off your skills as a developer", "see what kind of extra features you can come up with").
Define "very significant", you read that brief and you think "an entire week of full-time effort"?
Yes. Because it's a vague unbounded take-home exam and when companies pull stunts like this, they're usually looking for candidates to go 'above and beyond'.
I've done take-homes before and have been rejected because I followed the prompt to a tee and only did the exact amount of effort they wanted, and was quite literally told that they expected me to do more (outside of the prompt). If you've done enough of this game then everything in the OP will be extremely familiar to you.
It seems like half of the commenters here believe they could implement a Golang TUI that sends and receives emails in less than 2 hours. Clearly they have never tried to build a TUI, let alone consider any kind of the issues that might occur between different terminal emulators (OS, Terminal emulator, Shell). The web is king for a reason.
The only explanation I can presume is that these people are mistaking TUI for a CLI app, which would be a fatal mistake.
Ever heard of charm.sh?
edit: commenters here are also missing the forest for the trees. If the candidate had done X, Y, Z to make a more compelling entry then it would be some other poor guy staring at a two line rejection of tens of hours of work. The process is exploitative.