“Explain what you’ve learned from users”
If you find yourself thinking "I haven't learned anything from users; I have a lot of assumptions about them, but I haven't talked to or observed the behavior of any," bingo!
Not that anyone in that position is ever so aware of it.
Even if you only thought of the idea yesterday, you can still talk to users. In fact, that's the first thing I'd do after working out the first version of most ideas: run it by some potential users to see how they reacted.
The Mom Test