For what it's worth, there doesn't seem to be anything obvious in the XML which would make zombies do more damage to poles specifically. I checked concrete poles and wood poles and there are no special XML tags on either of them (compared to, say concrete or wood blocks).
I tested with two zombie Boes and a wooden block (225hp) and a wooden pole (225hp). In a few trials, both blocks were destroyed at essentially the same time, although when the wooden block is destroyed it 'degrades' into a wooden frame for an extra 50hp which Boe then had to destroy also. So the net effect was that the pole-blocked Boe escaped a bit sooner.