After a heated discussion between Manager and Senior developer

Manager: Just to let you know, I am not a fool.

Me (as a junior developer): I understand.

Statistically, the odds are pretty in favor of the manager actually being quite a fool without any business domain knowledge and not a single hour of coding experience.
But you certainly did everything right: Never tell them the truth - most managers also don't honor honesty at all...

"Show, don't tell" as the saying goes.