It is: "There should be one-- and preferably only one --obvious way to do it."

The core of that statement is "There should be one obvious way to do it" - there's no "only" there, it just says that when you need to do something, there should be an/some obvious way to do it. Then, preferably, that should be the only obvious way — though of course that doesn't preclude there being many other less-obvious ways.

With string interpolation we've certainly now got multiple ways to do it; that doesn't violate the principle. I agree that at least two of those are "obvious" (f-strings and .format()), and you could argue that %-interpolation is obvious too — but none of that is in violation of the principle that there should be an obvious way to do it, just of the preference that there's only one.

Finally it's worth remembering where this idea came from: it was in contrast to the perl mantra of "there's more than one way to do it", and a reaction to the resultant confusion frequently experienced when reading someone else's perl code — this was python saying "we're not perl, we value clarity and comprehension". I think that much of that problem was bound up in perl's syntax choices, and that in the cases in python where there's more than one way to do it (say, string formatting, dataclasses/attrs/namedtuples/etc.), it's usually pretty obvious what machinery is actually being used. When was the last time you looked at a line of python and said "I have no idea what the fuck is going on here?" That was a frequent experience with perl in the heady days of the late 1990s.