Earlier quoted context omitted.
Yes, exactly, and Unicode has more encodings than UTF-8 so stating it explicitly makes clear which encoding of Unicode that is. Writing only ASCII would not be relaying on Unicode, but on ASCII only. IMHO a bit shortsighted today.
Using UTF-8 but only the ASCII bits, and using ASCII are the exact same activity, however. I don't see how the first is any different to the latter, other than which document you look up to tell you that 00110000 is "0". The difference between an "ASCII subset of UTF-8" parser and ASCII parser is whether the error message on encountering a high bit set to 1 is "Invalid character" or "Character not in permitted ranges…
Which was my point. The encoding might be different. For ASCII 7 bit is fine, for UTF-8 and only ASCII, 8 bits are required and, as you also already point out, the high bit must be set to zero with it.
So the point is to name the encoding (UTF-8) of the ASCII characters.
And the input was portable code, not user input of a program.