Their point is that software is written so that it's only barely fast enough on machines much more powerful than a PineBook, which makes it more difficult than necessary to use perfectly functional machines that happen to be slower than average.

The problem is that fast software is harder to write than slow software (there are trivial transforms from fast to slow, but not vice versa). Thus, each generation back you expect your software to run smoothly on, represents more effort (or at least, more care) on the part of the author. We should expect software to be "just fast enough for the average user" essentially by natural law.