“Amazon EC2 Spot Instances let you take advantage of unused EC2 capacity in the AWS cloud. Spot Instances are available at up to a 90% discount compared to On-Demand prices.” https://aws.amazon.com/ec2/spot/
As a monopoly Google could sell the same capacity YouTube is using on Google cloud services and YouTube could then buy that capacity. If YouTube was treated as any other customer it’s fine, if YouTube got preferential treatment it’s not fine.
I doubt anyone is going to take them to court over this but it is likely illegal.
The problem comes from Alphabet having cheap compute left over from their search monopoly. In 1982 AT&T was broken up because it was using its local monopoly when competing in the long distance market. Again for 99% of companies it would have been fine, but monopoly = special rules.
What in the hell is "market rate" for compute? Google bought a zillion CPUs and hard drives and put them in data centers that they pay for. There's no "market rate", they own the infrastructure. They can do whatever they want with the equipment they own. A division using another division's spare compute capacity isn't in any way an anti-trust violation. What you're saying makes no sense at all.