I honestly don’t understand how that’s offensive. It’s just a metaphor, and a pretty good one. Why are you taking it as some kind of value judgement? Do you feel like people who don’t know something are lesser beings?

The book was originally written as a metaphor for understanding a fourth spatial dimension, and as far as I know the metaphor is generally taken as helpful, not demeaning. The flatlanders are never described in an unflattering way.