Enzymes are specific to a particular molecule, or class of molecules with a particular pattern. A PEI buildplate is not getting eaten by the proteases in a dishwasher tablet. The reasons you’re not supposed to rinse things before putting them in the dishwasher are:

• most dishwashers have sensors to detect how much material is ending up in the water, and if things have been rinsed, it can mislead them into thinking the load is lighter than it really is.
• dishwashers replace some of the dirty water part way through the load, and the enzymes are more soluble than the dirt, so if there’s not much food residue for them to stick to, they can end up getting rinsed away part way through the cycle.
• it uses water and your time to rinse the dishes first, which is a waste if it doesn’t make them end up any cleaner.