This is a generally known part of the history of mathematics.

> Nobody ever believed it to be a "fundamental mistruth"

https://math.libretexts.org/Courses/College_of_the_Canyons/M...

"Lobachevsky [mathematician contemporary of Gauss, who claimed parallel postulate was unnecessary] was relentlessly criticized, mocked, and rejected by the academic world. His new “imaginary” geometry represented the “shamelessness of false new inventions”"

Further, many claimed premature success in finding logical contridictions in geometry lacking parallel (Euclid's 5th) postulate; which meant they believed a 4-postulate geometry to be fundamentally false.