If she would learn haki their she would have it in PH you genius. In the flashback we see how Robin learn technique not haki. If she would learn haki back then Luffy would mentioned Robin alongside with Zoro and Sanji. The L suit you well.
Sabo's Dragon claw technique is haki based if she taught Robin then obviously Robin would know Haki.
But like I said, you can't read... you'd rather believe Robin got magic haki lol
For all we know, it might turn out to not be haki