No, it wasn't.
There were estimates of the Earth's radius spanning back to the ancient Greeks and before ... not the sort of thing a Flat Earther would be likely to think of.
Anybody seeing a sailing ship sailing over the horizon knew that the Earth wasn't flat ... and, in fact, that's precisely how the old estimates were made.
I do realize that. It is why I said most scientists, not all scientists. Most did believe the earth was flat, or at least said so in their writings. There were some brave souls that risked everything when pronouncing the earth was round, but they were definitely in the minority.