True, Haki wasn’t visible pretimeskip, but that doesn’t change how it functions. Even when Haki was invisible, Oda still showed its effects clearly such as physical blows landing on Logias, weapons being coated, or Haki clashes creating force. There has never been any indication that a purely elemental attack has ever interacted with Haki the way physical attacks do.
Oda did not show haki coating in Kizaru lasers, Big Mom Prometheus, Kaido Boro Breath after TS.
“Absence of evidence isn’t evidence of absence” only applies when there’s a reasonable expectation it could exist unseen. But in this case, there have been dozens of opportunities for elemental users to use Haki offensively Akainu, Enel, Kizaru, Sabo, Marco, etc. and none have ever demonstrated it. That’s not just absence of evidence; it’s a consistent pattern across hundreds of chapters.
So until Oda shows elemental Haki coating, saying “it can be done” remains speculative. Saying “it hasn’t been shown and there’s no indication of it” isn’t headcanon it’s sticking to what the manga actually establishes.