Interesting! But @drewisfat ’s point half stands, due to this (in bold)?
And most importantly units that fullfill these criteria are considered targets, but don't receive damage, therefore reducing the effectiveness of collateral damage, just like Mr. Cairo already discovered
Units with Type NO_COMBATUNIT (there is none in the game)
Collateral immune units
Damaged units that are under the damage limit. (but it's less likely that they are selected because of their reduced HP)
Oh, I looked up how the probability for targeting works. It's just a random number multiplied by the units HP. I thought this was added to other contributing factors, but nope. In that case, a full health catapult would be chosen with the same likelihood as a full health lb while an axe at the 50% collateral limit would only be chosen 25% of the time over the lb. So cats are much better for this purpose.