In that case you have more than one implementation, or at least a reasonable expectation that it will be used. I don't have a problem with that.

My comment was regarding interfaces used internally within the code, with no expectation of any external use. I wrote from a modern Java perspective, with mockable classes. Apparently interfaces are used by .Net to create mocks in unit tests, which could be a reason to use that approach if that is considered "best practice"

90% of single-implementation interfaces (in Kotlin on Android projects I've seen) are internal (package/module private, more or less.) So no, they are not meant to be extended or substituted, and tests are their only raison d'etre (irony: I've almost never seen any actual tests...) This is insane because there are other tools you can use for testing, like an all-open compiler plugin or testing frameworks that can mock regular classes without issues.

An interface with a single implementation sometimes makes sense, but in the code I've seen, such things are cludges/workarounds for technical limitations that haven't been there for more than a decade already. At least, it looks that way from the perspective of a polyglot programmer who has worked with multiple interface-less OOP languages, from Smalltalk to Python to C++.