A modern C program doesn't compile in old C compilers. That doesn't mean it's not C. You can change tons of ergonomics and keep the language the same. Or you can keep the spirit the same (which in a language are its core semantics and concepts).
>I believe GP simply didn’t develop their argument effectively.
GP just casually said it's a whole new language in the casual, everyday sense, that a e.g. you can say "it's a brand new car now" after you redid the interior and did some repairs on your 10-year old car.
(And I got in with the Ship of Theseus thing to humor the objection -- not that I really consider the GP statement to have to be taken at face value).
Same language doesn't mean you can necessarily speak it without learning the new words, forgetting some stuff that went deprecated, learning some syntactic sugar and new features. If you were born in 1400 and lived to today, you'd be going from speaking from Medieval French to modern French with hardly noticing the difference...
So? C# 1.0 didn't support 90% of the cool features of C# whatever it is now, not even LINQ, the bread and butter of C# programmers.
Java up to 1.4 didn't have generics, closures, streams, modules, and tons of other stuff besides. Soon it will be getting many more.
Is it a different language for that?
Thing is, adding and/or removing features may have an impact on the implementation, can potentially introduce subtle changes to the semantics of the existing constructs and therefore their behavior at run time, have general performance implications (e.g. due to a change in what optimizations are possible), etc. Is it "the same language"? Not quite. It is a "super-language," at best. If it's 100% backwards compatible, that is (which is not guaranteed).