Product A: dominant search product
Product B: non-dominant advertising product.
If you want to use product A, you need to also use product B. That alone is sufficient to create an antitrust violation. It's almost literally the textbook example of an antitrust violation.
But Google didn't stop there--they required exclusivity for product B for several years, and even after that ended they still required premium placement. Either way, they just compounded the severity of the antitrust violation.