It seems a little strange that
(by themselves) imply agreement (something like 就是). The textbook 标准教程HSK4上 has some examples, e.g.:
I'm expecting there's an underlying reason for this I'm unaware of. Perhaps it's a rhetorical question like "Is it not?"
Question: Why do 可不 and 可不是 by themselves imply agreement?
It seems like they should both mean "but no" (i.e., disagreement). Indeed, they also mean this, e.g. (sourced from YouDao):
This will not be easy, however.