The question is a request for strictness. Fine, I'm going to play along.

The phrase "for computer enthusiasts and power users" is not equivalent to "*only* for computer enthusiasts and power users".

[The page containing the phrase][1] is entitled "What topics can I ask about here?". If it was entitled "Whom is this site for?" then I would expect it to answer this exact question *comprehensively*, so from the lack of descriptions other than "computer enthusiasts" and "power users" I would conclude the site is only for them.

Instead the page is about topics, i.e. about questions, *not about users*. The sentence "Super User is for computer enthusiasts and power users" is just a preamble and I see no reason to assume there is (or ever supposed to be) an implicit "only" there.

You pointed out that "power user" is reasonably well defined, reasonably well measurable; and "computer enthusiast" (allegedly) is not. What difference does it make? Nobody verifies if any user is a "computer enthusiast" or a "power user"; we have no means to do so even for the well-defined term.

What matters is being on-topic (for questions) and answering the question (for answers). Even if one is neither a "computer enthusiast" (whatever it means) nor a "power user", but manages to write a suitable question or answer, then it fits.

For me the phrase in question is fine as it is.

  [1]: https://superuser.com/help/on-topic