since Dabi fights with flames that are much hotter than those of endeavor or shoto. Isn't it a real possibility that he could be heavily resistant if not immune to regular fire.
Endeavor’s Fire is not regular Fire. Endeavor’s Fire is Hell Flame. It took Shoto the first half of his first year training to even unlock the ability to use Endeavor’s
base-firepower, which can instantly slice through metal like nothing and wound Shigaraki who is as durable as All Might.
Endeavor also stated that Touya had inherited his mother’s weak constitutions, meaning Dabi not only is not “resistant or even immune to flame”, but Dabi can’t even handle flame as well as Endeavor can. More on this below.
Dabi literally knows prominence burn, the strongest flashfire fist move. This implies that he not only knows flashfire fist, but that he's actually mastered it.
Prominence Burn is only one aspect of this fight and a fairly unimportant one at that. Even if Dabi had Prominence Burn greater than Endeavor’s Prominence Burn, this basically means nothing when Endeavor beats Dabi really badly in every other category.
Endeavor is the number 1 Hero who has solved more cases than anyone else in history, including All Might. Meaning Endeavor has probably defeated more villains than even All Might. The extent of Dabi’s experience is that he committed petty-crimes like robbery for a few months before joining the league. Endeavor was the number 2 hero when he turned 22 lol. Thanks to this experience, Endeavor is constantly operating in “Ultra Instinct” Mode, meaning his mastery of his quirk is so high that he doesn’t ever hesitate or second guess his decisions-he makes the optimal decision in every scenario, and he is capable of fighting on auto-pilot should he choose to use his brain on other things.
Dabi on the other hand, had to verbally stall against Tokoyami (who should be weak to Dabi) as every single time Dabi let’s off a single fire blast, his body suffers such atrocious recoil that he has to take a break for several minutes before he can fight again. The fact that Dabi didn’t instantly defeat Tokoyami is pretty shameful when Tokoyami should be excessively weak to Dabi. This is the Dabi you think should be immune to Endeavor’s flame lol? The one who can’t let off two fire blasts within a minute of each other or he’ll kill himself?
Endeavor is also much more powerful than Dabi physically. Endeavor tanked multiple blows from Awakened Shigaraki when Shigaraki had all of his quirks. Enji is a hulking mass of muscle who constantly trains himself physically, compared to Dabi who is physically frail and has weak constitutions. Endeavor’s own assessment of Touya confirms this. This is the entire reason Endeavor threw Touya aside in favor of another child. Endeavor also has fighting strength and can deliver seriously powerful punches and kicks enhanced by his quirk.
All Dabi does just shoot basic flame blasts every 60 seconds or so. His combat abilities don’t even seem to exceed Shoto levels of basic.
Endeavor is also brilliant. He instantly assessed the extent of Hood’s quirks, and instantly figured out the strengths and weaknesses of Bakugo’s and Midoriya’s quirks after only seeing each in action for like one mission.
Dabi is pretty smart too, he was able to figure out that his only chance to defeat an opponent who he should have instantly neg diffed is to stall for 5 minutes between spamming UA first-year level attacks and then failing to neutralize his target anyway.
Dabi had good potential but he is essentially a scrub who Endeavor threw aside the instant he realized that Dabi would never overcome his own horrific weaknesses.