Popular sites were referenced for the first themes as a means of sticking to well recognize design patterns.
Using these as archetypes is a convenience. But it may also mean that one or more of these initial themes may have to be removed from this source repository later, if legal objections arise.
So, use at your own discretion.
Webster defines an "archetype" as "the original pattern or model of which all things of the same type are representations or copies" or "a perfect example"
That kind of question is way beyond my pay grade, but this article seems to say that there is nothing to worry about, all HTML/CSS is original to these themes, virtually none is copied from other sites.
There may be exceptions with HTML/CSS snippets copied from tutorial sites or from stackoverflow, for example, but that would hardly make these themes unique or vulnerable, at least as far as copyright goes?
These themes are starter themes, made to merely prove out the idea of theme-ablity for Rocket or other 11ty sites.
They are such poor fidelity themes that it hardly seems likely that they will be accused of being "copies". More like "inspired by"
It is unlikely that you will be using one of these themes as provided. As fundamental as they are, you will be doing at least some degree of new CSS and other revisions to get your site exactly as you wish.
In that setting, it won't much matter what happens to the source, because you've already forked.
If it bugs you to use archetypes, you can still start with one of these to learn some Rocket theming basics, then fork past any recognizability.