Glycemic index measures the time course of glucose levels in the bloodstream.
By definition it's impossible to be slow as measured by glycemic index but fast in terms of delivering glucose to the muscles.
And no, waxy maize itself is not going to be delivered to the muscles. It will be as glucose that it gets there.
The two claims are just totally inconsistent. One has to be wrong, it seems to me.
As for the argument presented to you and I suppose the target audience for waxy maize of " the igh molecular weight of the starch (which is heavily branched) makes it very low in osmolarity which in turns makes it pull water with it through the gut" this makes no sense.
It's words that string together and sound scientific perhaps, but make no sense.
It is low in osmolarity for any given amount of total glucose unit content (upon hydrolysis) because that is a function of the number of molecules in solution. One starchy molecule per volume of water has lower osmolarity than many glucose molecules, so long as the starch has not hydrolyzed.
That part is true.
But this does not drag in water with it.
Even if it did that would not speed delivery to your muscles.
What it does do is allow water to be absorbed more easily, which is a good thing for hydration. But it is not a mechanism yielding faster delivery of glucose.
Again, I haven't looked into it specifically. I have just not seen an argument for waxy maize being as fast as glucose that made a lick of sense, and the claims of it being low or moderate in GI but "fast in practice" (or another phrase to describe it) are contradictory.