Their contract with Triplebyte stipulates that companies that use their service aren't to incur additional technical interviews, and according to the Triplebyte representative I talked to, apparently the company has legs to enforce the contract if a candidate informed TB of a breach.
When companies try this, and pretend to be confused when there is push back, it's because they got caught with their pants down trying to breach an expensive contract.
It was my experience that every company, big and small, that I interviewed with through TB did on-site technical interviews anyway. In the end, the value-add of TB was that you could filter out many of the companies on the platform because of how cavalier they were to dance around their contractual agreements with their recruiting agency.