So, did I just learn this correctly, that cannot be frozen is pointless for PVM casters?

Technically yes, cannot be is not necessary for pvm casters. Frozen does not slow your casting at all.. the only reason to take cbf for a pvm caster is if you do a lot of kiting and you do not have any version of Tele to keep safe distance from enemies, in which case frozen would slow you down and allow enemies to catch and kill you.

if yu don't have tele they will kill you either way

only downside of not havin cbf for a sorc is to get frozen and you go to town.

Does it slow attack speed also?

yes