Roughly: They're not PCs. For one thing, they all have limited battery life. For another, not everyone can afford brand new flagship phones, so folks who keep cheaper phones for long enough can easily find CPU and RAM to be stretched as new app versions start to demand more of the hardware. Conserving resources is beneficial in general, and especially so on mobile devices. Every little bit helps.
Why I consider smartphones constrained is unimportant here, though. The question isn't whether GNU libs/utils can run on a smartphone. The question is why the person above said GNU specifically.
I'm trying to suss out whether they are aware of some special case that would make GNU better than e.g. musl on a phone, or were just repeating a bit of that "GNU/Linux" rant that was (incorrectly, I think) attributed to Stallman. If it's the former, it seems like information worth sharing.
Thanks for the detailed explanations. I wrote GNU because I meant it. I'm talking about running an original desktop OS on phones, which unifies the software stack and allows to have a full desktop in a pocket. It's much more convenient to have the same software running everywhere. You can connect a screen/keyboard and get a full desktop [0]. Firefox on my phone is a desktop one without a rewritten codebase. You can use all GNU utilities in a terminal on the go. If you are talking about saving resources, then why do you think we need several devices when we can have just one? All existing desktop software doesn't have to be rewritten from scratch but should "only" be made fit for both large and small screens. Doesn't it save resources?
Concerning the battery life, we need to improve it for laptops, too, anyway. Also, currently, it already allows to have a one day of use, which is mostly sufficient.
They have more memory and processing power than a normal computer from 20 years back, even 10 years back.