I remember being told it, and my memory is freaky good. I think the assumption that it's the Mandela Effect is pretty weak here. This isn't documented in the same way as the death of a famous figure, or a brand logo, or a book series. You don't have the counter evidence, and your argument assumes that teachers are perfectly knowledgeable and rational.

The era I'm referring to is mostly the '90s, and a bit of the early 2000s. The first iPhone, and the normalization of actual screens, was the late '00s. Even flip phones with basic screens didn't become common until the mid '00s. The technology you're talking about definitely existed, but wasn't that common. Cell phone technology, and adoption, really skyrocketed from 2000-10.

And teachers say nonsense all the time. I have a distinct memory of second grade, learning about the prime meridian and equator, and that if you cut the Earth along either one you got halves called "hemispheres". I asked what you get if you cut along both the prime meridian and the equator, and my teacher said "Well, you get a broken globe", and then continued on.

Teachers are just people. They don't predict technological trends, they say things based on their past knowledge to get kids to engage with the material. Assuming you've never tried to teach children things for an extended period of time, it results in saying a lot of stuff that's more rhetorical than factual.