When I said “flat earth,” you immediately assumed that meant the land and mountains must also be flat.

That’s actually a strawman fallacy, you built a simplified version of the idea and then argued against that instead of what I really meant.

It’s also a form of jumping to conclusions, because you took the word flat at face value without asking how the theory actually works.

The better way to approach it is with curiosity:

“What do flat earthers mean by flat?”

“How does the theory explain mountains?”

That way you’d be questioning the idea itself instead of arguing against your own assumption of it.