Fair enough. But the theory is widely accepted. Of course, that's not the best argument. As to the realism of the events painted on Greek pottery, that's always a good discussion. But it's some of the best and often the only evidence we have for some things, including how you hold a spear in a hoplite formation.
And the evidence for riders using the grip during the Middle ages is mostly in illustrations in period books, usually bibles. I think the bayeaux tapestry also shows riders using their spears overhand, could someone confirm that? Overhand vs underhand is a really touchy subject, so lets be careful that we don't completely derail the forums over it.
However, back on topic, using an overhand spear thrust to thrust upward, such as using it from horseback to stab up at at the rider of a camel, (the scenario in question) would be incredibly awkward. However, switching to an underhand grip and thrusting up wouldn't be much of a problem.
Cheers
Kvedulf