I fully agree with you - the practice of price discrimination is illegal under the Robinson-Patman Act and Google should be penalized for violating this law if they have been found to do this. I am only trying to push against the notion that Google (or any company) should be broken up just because they are big. This is the nuance that makes these discussions important IMO
My position is basically that enormous market power is like a ticking time bomb. It's not going to work to try to patch over it with specific prohibitions like the one you mention, because big companies will always find loopholes, use their market power to exploit them, and use their wealth and "too big to fail" status to drag out any attempt to enforce the rules. It is an endless game of whack-a-mole that can never work. It's better to just limit the total power of companies to do anything; doing so will also limit their ability to do harm of any kind.