Oh really? How does Sanji counter death enjambre without getting damaged?
Speed-blitz Diamante when he takes out the canisters and before Diamanate un-waves them. Blitzes Diamante as the confetti is shot in the air, before Dia releases his power to make them into the iron balls. Or simply blitz Diamante as they are falling. Seeing as Robin and Rebecca had time to stand around, talk about the balls, then Robin had time to sprout arms to guard against the balls with freaking SUNFLOWERS, Sanji can easily blitz Diamante during this period of fall before the balls get close to him. Diamante's Death Enjambre is full of openings that anyone with respectable speed can exploit, and seeing as Kyros, Rebecca, and Robin could deflect/block them, Sanji definitely can too.
It's flamability is dangerous for the surrounding opponents/targets not himself. We've seen him multiple times using his flamable stuffs up close and never damaging him while at the same time damaging the targets. So if anything using diable jambe technique would put Sanji at a disadvantage, not Trebol
Wouldn't this work both ways? Sanji is literally setting himself on fire whenever he used Diable Jambe and Hell Memories, so I doubt those same flames being pushed back on him is gonna hurt to that degree where it makes any kind of difference. Sanji blitzes Trebol with his hardest kicks, the flames that are already on Sanji without hurting him expands outwards, and Trebol is hit with the force of Sanji's legs while Sanji gets maybe some minor burns. That doesn't justify mid-diff.
That was against a Vergo who was using Sanji's kicking fighting style without using his own(brawling and bamboo fighting style), let alone his hardening. It's like saying Sanji can beat Luffy in brawling if he manages to outkick him in a kicking fight. Besides, Sanji himself admitted the fight would have gone bad for him if it continued
Sanji was also, at this point, in a body that was so weakened, a blow from Nami caused him real pain. As in, not pain that's 100% comical and gone in the next page, but actual, acknowledged "This is real pain" pain. It's obvious that Sanji's body wasn't at its best when he clashed with Vergo, so that's an inconclusive fight.
As for Sanji's comment, Sanji never said it would have gone bad for him. He said "If we went on..." and never finished his train of thought, which could mean any number of things, especially when you consider that the poisonous gas was closing in on them. It can easily mean "If we went on, the gas would have gotten to us..." or "If we went on, I'd be here forever before I beat him."