I'm guessing they're thinking of Extended ASCII (the 8-bit one that's actually multiple different encodings, but the lower half is shared with 7-bit ASCII and s

I'm guessing they're thinking of Extended ASCII (the 8-bit one that's actually multiple different encodings, but the lower half is shared with 7-bit ASCII and so that part does fit in UTF-8 while the upper half likely won't if the message actually uses it).