Onoki was probably born with the dust release kekkei touta and was taught how to use the dust release: detachment of the primitive world technique by Mū thus explaining Onoki's statement without saying Kekkei genkai can be learned.
Madara also didn't use fire release in the first panel does that mean he doesn't have fire release until the second panel or that they were just not using ninjutsu in that exact moment of time.
As for Hashirama being the only Senju with Wood Release then perhaps he was born with a random mutation after all their would have to be a first person to have other Kekkei genkai like ice release or lava release.
Madara also didn't use fire release in the first panel does that mean he doesn't have fire release until the second panel or that they were just not using ninjutsu in that exact moment of time.
As for Hashirama being the only Senju with Wood Release then perhaps he was born with a random mutation after all their would have to be a first person to have other Kekkei genkai like ice release or lava release.