A number of the 8-bit sets that embedded the ASCII 7-bit set were informally referred to as ASCII especially during the 1980s; this may have been encouraged by microcomputer BASIC interpreters using the ASC() function (named for ASCII) to return the character code of a character in the system code set, which was often an 8-bit set that embedded ASCII.

You would be surprised. Referring to any of the most common 8-bit encodings as "extended ASCII" or even just "ASCII" has been very common in my experience.

Yeah. Most people (well, back in the DOS days anyway) who don't know what a "7-bit encoding" is would refer to code pages as "ASCII". I'm sure I did that hundreds of times myself (I didn't know better until I was what, 20?) and I've seen it thousands of times, not exaggerating. Just look at how many "[extended] ASCII games" there are.

I know a lot of DOS people used the term "extended ASCII" to mean specific 8-bit encodings whose first 7 bits were ASCII, but what "extended ASCII" meant depended quite a lot on the context. I came from a different background and it was very clear to me that ASCII was a 7 bit encoding that was almost always stored in an 8 bit byte (I can only think of a few instances in my whole life where I saw bitstreams of ASCII). So that last bit you could do whatever you wanted with.

Having said that, I clearly remember arguing with people that encodings with the eighth bit set were not part of ASCII. As you say, there were many people who didn't understand. My guess is that the GP never interacted with those people. Especially if you were around pre-DOS I think it would be easy to do.

I'm guessing you're just not old enough to have heard anyone refer to extended ascii?

https://en.wikipedia.org/wiki/Extended_ASCII

If you used Macs in the 80s and 90s and shared files with PC users, you heard about it all the time, because high-ascii characters would map incorrectly across the platforms.

I mention it because I've seen it happen multiple times over my career.