Eh sometimes brute force surpasses haki so I don’t know
and also a thing to remember, it took ray 1.5 years to teach a prodigy basic haki. I don’t think there’s enough time unless Oda introduces a time chamber
I don't think having someone teach you the basics of haki specifically is the only way to learn it, you can learn it through hard battles as you improve it unconsciously. So rather than thinking they would need to learn it in a few days/weeks, we might have to consider that the learning process started for them 2 years ago during their training, but that wasn't enough time for them to master it like it was for the M3. So the learning process continued after the timeskip.
Don't forget Usopp unlocked CoO without anyone teaching it or even explaining it to him.