It's a valid theory, but no one really knows.
When you look at Kabutops and Genesect, they resemble each other greatly. To me, Genesect looks like a Kabutops with purple armour.
>Over 300 million years ago, it was feared as the strongest of hunters. It has been modified by Team Plasma.
Pokedex entry for Genesect
Here it says that Genesect was "modified." My theory is that they modified Kabutops, adding the cannon on its back, to make it stronger.
Kabutops and Genesect both have a similar stance, as well as an extremely similar body shape.
However, this is just a theory, so people may have different opinions. There is not written proof that Kabutops is Genesect and has been modified, although it seems very likely.
So, judging by all this, I would say it is much more probable that it was Kabutops than Armaldo.