For any instrument, there is an implicit assumption that you've got the basic physical abilities common to all people in reasonable health.

I will agree with that.

This doesn't change the fact that the guitar is a very easy instrument to learn for anyone who does have those basic physical abilities.

And I will disagree with that. The guitar is NOT an easy instrument to learn. I have tried on three different occasions in my life to learn some basic guitar and simply could not get away with it.

I can play recorder to a reasonable standard, am learning flute and anglo concertina and am making good progress in those and have not found the basics too difficult, though I do mainly treat the anglo as a melody instrument, rather like the Irish do.

OTOH, I have a friend who is very much at home on guitar but who finds the whistle difficult. Different instruments suit different people and it is not about musical ability but a combination of your physical and mental make up.