so far only the yami yami has canceled another devil fruit, yes most probably even if neither of them had Haki magma would have won, but I would say a Logia of Ace's caliber does not go down in 1 hit without Haki, that's jsut my thoughts on it, you have your own, which is great, and might as well be correct, but sadly we eill never know unless Oda answers this specific question
but I should mention that the main reason I think Akainu used Haki is because Haki was very ... hmmmm ... ''loosely'' shown pre TS, I would imagine most named characters at MF used it but obv Oda had not yet fully figured it out, I think that an Admiral using Haki when punching the son of both Roger and WB is just common sense and a given