[triffid]

I am trying to wrap my head around this statement: "the hydronium ion is the strongest proton donor in aqueous solution", from my chemistry textbook when reading about acids/bases.
Is this because any acid stronger than H₃O⁺ completely dissociates, and so there is no more acid left in solution (it is now all conjugate base and hydronium ion)?
I've had a look at pKa values for acids weaker/stronger than hydronium in water, but there is a problem of finding the pKa of the ion as it seems to be debated.
 From what I have read on this question: https://chemistry.stackexchange.com/questions/9847/what-is-the-pka-of-the-hydronium-or-oxonium-ion-h3o, the pKa of H₃O⁺ is either 0 or -0.7, which fits with HCl, a stronger acid, having a pKa of -6.3 and HF, a weaker acid, having a pKa of 3.17.
The post also goes into some other chemistry that I am not up to the level of that I am stumped about, and I was wondering if there was a simpler explanation for the statement that I have somehow missed?
I'm still uncertain in areas such as how H₃O⁺ can be a stronger acid than acetic acid in solution for example, so maybe I don't have enough background knowledge to properly understand the statement yet.