I am unsure where you are finding that info, but here in the USA, Zosyn can be given IVPB with LR, however the warnings are for reconstitution, not administration. LR will cause Zosyn to degrade after 8 hours.
But, if you have reconstituted it with sterile water and have an IV infusion of LR, that piggyback will be given over 1/2 hour and the contact with the LR or RL, infusion is minimal.
What you do have to watch for when hanging LR is assessment of that pt's liver status and do not give LR to pts with a pH of 7.50 or greater. (the lactate ion is converted in the liver to bicarbonate which will further compound the problem)