Indeed, liberals cannot say what is or isn’t politically correct speech. Who can know what is or isn’t politically correct speech?
The entire point of political correctness is to avoid actual debate on facts. It is a tactic, not a philosophy. Quick example: a liberal friend and I were discussing inner city problems. I stated that the breakup of the family and high number of illegitimate children were the primary problems, and that fleeing industry, crime, drugs, etc., all flowed from this point. He immediately began a diatribe about how the children weren't illegitimate, they had as much value as any other children, etc., etc. The point was to avoid the actual debate of issues. This is the primary point in the terms "undocumented worker" as opposed to "illegal alien," and on and on.
The purpose is to keep you on eggshells, fearing that one term might offend. The point of political correctness is to keep you on the defensive, avoiding factual debate, and getting submission prior to beginning discussions.