Monks and petards cannot enter Siege Tower

If this is meant to be that way for balancing reason, then it should be reflected in the Tech tree.
For Siegetower the description merely states that it can’t garrison “mounted units”. It’s somewhat intuitive that it can’t garrison Siege perhaps, but one might think that the Petards count as infantry and would give it a speed boost (which would be hilariously broken and definitely something to be considered).
But it can garrison villagers, so why not monks?
Figuring out the rules of the game shouldn’t have to rely solely on trial & error.