Counter-counter argument: A person creating a personal homepage has no obligation to cater to any particular audience. See, for example, jwz.org [NSFW when linked from here, very much on purpose]
You are right that nobody is beholden to creating pages that can be viewed on any device. However, it is not unreasonable to say that a person has mismatched interests when they say they care a lot about quality, interoperability and values in general, but then don't care about making the document explaining this viewable on most devices.
Yes, ultimately it's not important. But if you only publish your website using Gopher, at some point you have to accept that you're interested by making cool things rather than the other things mentioned.
That’s irrelevant. Most of the Web users are using mobiles, so if you decide to set a website up, the very basic thing is to ensure it’s readable on mobile. And since plain HTML is already readable on any device by default, it would be quite strange to voluntarily make it unusable on these devices.