I was arguing generally against permissive licences and in favour of copyleft, without pointing out a specific one, but as a matter of fact I do think that GPLv3 would be a much better choice for this project. (The thread’s OP did name specific licences, though.)

> Why can't this project use whatever license they like?

Why they can’t or why I argue they shouldn’t? Technically they’re able to choose any licence they like, and I argue it should be a copyleft one because I think preserving the four user freedoms is important and not accomplished with permissive licences.

> If not all FOSS project need be GPL3 but this one should, what are some examples of ones that you would be fine with being MIT?

All of the various MIT licences are similarly permissive, right? In that case, I think they make a decent choice for small and relatively simple programs which do not present a great amount of work and could be replicated or imitated without much effort. Programs like True, Hangman, Cowsay or short userscripts.

But if you have a relatively large program, or a project where a great number of such smaller programs work together, I think they ought to be released under a copyleft licence. A new kernel is a good example of such a program, as are core utilities, terminal emulators, media players, web extensions, complex games, etc. So Maestro, seL4, Redox, GNU, Terminology, VLC, mpv, uBlock, Minetest and so on.

I think weak copyleft licences (such as the MPL or the LGPL) can make a great option for programs with a good ethical reason to maximise their usage (including by non-free software). In my opinion, there's seldom a good reason to abandon copyleft altogether. (And as that may imply, it is my view that copyleft should be the default choice. When in doubt, protect your users’ four essential freedoms.)