It's a good point. We have seen discrepancies with descriptions before, but at least it's one piece of evidence towards their intention.
I thought it may have something to do with the formation as I believe the bonus is supposed to represent the fact that elephants and chariots are less effective against loosely formed skirmishers who can jump out the way and throw spears at them. The formation for most peltasts are 1.6, 2, 3.2, 4, 3, square whereas Illyrian mercenaries are 1.2, 1.2, 2.4, 2.4, 4, square.
There are however two other units with that formation (Heavy Peltasts and Mercenary Peltasts) and they both still get the mount bonus.
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