Hey, before you go start gaslighting strangers that their memories are false (I also remember hearing the phrase in school in the 80s), consider a few things:

First, while calculator watches may have existed, I don't remember seeing them in my childhood. They weren't universal.

Second, teachers are adults, kinda set in their ways. Brand-new 'toys' aren't going to change their thinking on a dime. I can very easily believe teachers continuing to use the phrase even after circumstances have changed (see also the other poster mentioning hearing it in the 00s).

Third, the fact that you can "prove" the saying wrong with your arguments doesn't mean that all of our teachers have thought it through the way you did (and I think all of us have had teachers where the response to this debate would be "Quit talking back! Detention for you, buster!" instead of "Oh, yeah, interesting thought...").