That's silly, Mark...
If TB is retreating to relative mobility to explain Equidae, then TB's frantic hand-waving has finally unhinged his reasoning ability. I say that not because relative mobility in a Flood is a silly idea (although it is), but because TB has pegged the end of the Flood at the K-T boundary! All his Equidae are post-Flood variation in only a few thousand years - evolution faster than any observed. He has totally forgotten how his own model (fails to) fit together.