What are you on about ? Luffy never gained any type of haki against Fodders.The only thing Luffy did ,was try to focus his already existing haki,that he was emitting unnecessarily outwards ,to a single point to create the barrier type. It is a technique. He didn't enhance shit.
In the TS they needed to gain haki and make it stronger and also become stronger physically and in all kinda of ways. They aren't going to learn haki or grow stronger in general ,by fighting fodders that they beat at day one....
Luffy against fodders = learning Haki, a new type of Haki.
Luffy against Doffy, Cracker, Katakuri = enhancing his Haki which he already has without developing new Haki.
Zoro against humandrils = learning Haki, not enhancing it. Enhancing against Pica, Killer, whoever.
What is so difficult to understand?
If want to be a Formula 1 racer and you don't know even how to drive, you won't learn it in by using a Formula 1 directly. You try to learn it from the basic and so you need a basic car.
Mihawk taught Zoro about Haki and Zoro practiced his control against fodders, basic and elemental "things". You can't expect him to go to a death battle to learn Haki. Like you can't expect it from Luffy.
Or were you expecting Luffy to learn the Advanced CoA now against Kaido in a death battle just after he was oneshoted previously? What would be the point? He would be oneshoted again and he wouldn't hurt him at all.