It was a retcon, it was quickly changed thought
she could’ve used it to fight Monet and defend herself against Kyros. Luffy also can sense haki and he didn’t know Robin had it
the best route is just her learning it while off panel
That's not how retcons work. Retcons don't literally fix gaffes, they're explanations that come after the fact. Dialogue changes between the magazine edition and published volume aren't changes to the story.
Luffy later mentions fighting Katakuri as if he was a logia and maybe that confuses you. But remember that he thought the same about Trebol in a similar situation.
ALL living beings have Haki, but there's no way to detect who can use it. When Law asks Luffy, Luffy isn't sure about Law. When Sentomaru sees Luffy use it, he's surprised. Robin likely isn't at a level where it's really dependable. She certainly wouldn't be able to block Diamante, hell Luffy couldn't block Hody. She only hit Monet once when Zoro was already fighting where now she's fighting for herself.