Renaming Interpolation, reworking the template creation signature to better support dynamic templates, and renaming args all sound like good improvements to me.
I confess that repeatedly using the “segments” variable name in examples was intentionally testing the waters before proposing bringing the “segments” attribute name over from PEP 501’s equivalent to the Template type.
There’s also a genuine inaccuracy in using “args”: since the constructor may insert extra empty text segments, or merge adjacent segments, the segment list may not be the exact same sequence as was passed to the constructor.
Edit: to be clear, I think reworking the Template signature would mean I no longer saw any need to revisit the protocol idea. The split constructor signature is a better solution to the same problem.