Yes and No. The Lower Resist curse is the only thing you can use to attempt to break a poison immune creature, so you won't be able to break many high ones. At level 40, Lower Resist has -67% all resist (including poison), so at 1/5 effectiveness against immune creatures it'll be able to break any creature with a poison resistance of 112% or less.
Most of the poison immune monsters have 100% poison resistance, 105% poison resistance and 110% poison resistance, so you should be right. There are, however, some with 115% poison resistance, 120% poison resistance and one with 125% poison resistance. There's still more higher than that (such as the megademon from uber trist / furnance of pain with 145% poison resist).
The data from d2data.net should be accurate with regards to resistances, since those are not affected by monster level (which is why d2data.net is not accurate for things like monster HP, damage, etc).