I kinda liked Gandalf going all Jacky-Chan on the orcs with his staff...
But, seriously, I was suitably impressed. Just reaffirms to me why I'm a fantasy writer.
(Though, as a caveat, Tolkien is the ultimate master of "Uh, where did that army that just saved us come from?" climaxes.)
And, on the Oliphant argument, I have to side with Entropy. Or, at least, the principle Entropy is arguing. Telling him to get his 'sense of reality' out of the discussion does fantasy literature a grave insult.
You see, fantasy depends on its ability to develop an internal logic different from that of our own world. I realize you know this, but with that realization one must understand that any time we allow fantasy to get away with irregularities simply because they're 'cool' or because they're 'magic,' we denigrate the genre and give into what the detractors commonly uphold as the form's gravest stumbling-block to legitimacy.
So, I would formulate two responses to Entropy's point. Either there is a rational story explanation (i.e., the Riders are no regular cavalry, and their horses trained beyond the scope of what is normal,) or it was an error in the story. That's not such a bad thing to acknowledge--any story will have its problems, and this one isn't so grand that it should be considered a serious flaw.