That's what "just say yes" means, as mentioned several times. It's not about just letting the player do whatever he wants. It's about letting the player attempt whatever he wants, without flat "nope, won't work" rebuffs.
You misunderstood me.
A better phrase would have been:
Instead of "Just say Yes", sometimes "Just say Yes", sometimes" Just say No".
It's not just about how difficult something may be and the PC attempting it or not.
The phrase Just Say Yes is from the DMG and also discusses whether the DM wants to allow something in his world.
Player: "I want to go to a Magic Item shop."
DM: "You ask around and are told there are not any."
is just as valid as:
Player: "I want to go to a Magic Item shop."
DM: "You ask around and are told of one in the bazaar."
The second one here is not necessarily preferable to the first one.