Likewise with nails in crucifixion.
The logic appears to be: we know that horses are the animal of choice to ride, so we should doubt Jesus actually rode a donkey into Jerusalem. By this logic, every detail of a story decreases its likelihood, independent of the evidence of what explanation is most likely given all the evidence.
The fallacy is mistaking P( Story1 ) for P(Story1 | Evidence) vs. P(Story2 | Evidence). The P( Story1 ) will always be low for just about every true story. P(Story1 | Evidence) vs. P(Story2 | Evidence), on the other hand, can point to clear winners and losers.