yes, because it is not said that antimatter gives momentum ability as well. Whole antimatter behavior is bad i think.
"Invert the attack power of the target creature (the creature inflict heals instead of damage)" this should mean that if creature can´t do damage, it can´t heal either. I think that removing momentum ability from antimatter would solve all the antimatter problems (except the ones with delay). But i dont know in which way it is programmed, so maybe it is not antimatters behaviour but shields behaviour, and if it is, then denying it from penetrating shields would be lot of work to implement
EDIT: Sorry, now i see we moved a bit from original topic. Simple explanation for reflective shield not reflecting poison is that reflective shield reflects spells that are TARGETED on you. But if you add poison you dont have to click on player, you simply cast it