Anti-Trust Law is about preventing dominance in one market being used to create an advantage in a different market. Google is accused of using its dominance in search to give itself an advantage in online shopping. Simply not selling your product to a competitor would not be anti-trust.
Google's defense could be "we don't have a dominant position" but they have 90% of the EU search market so that's a loser. It could be, "we don't manipulate the search results to give advantage to our shopping sites", which would end up requiring them to give the code (and history) to the court for analysis. Or it could be a lot of technical legal wrangling which is probably what will happen. It definitely WON'T be "you're anti-trust laws are stupid".
An analogy, if I own 90% of the billboards in a state I can't refuse to sell billboard space to Jack's Auto Repair just because I also own John's Auto Repair.
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JimFive