Except, of course, that the companies who work with TripleByte probably trust them a LOT to provide great/qualified candidates. Most companies don't trust recruiters very much at all-- some aren't terribly motivated by anything other than closing as many deals as possible, and the vast majority don't know anything about coding. And many/most of Triplebyte's companies probably don't work with recruiters.
Why do you think failing a tech screen with Triplebyte would exclude you from top companies? You could still apply to those top companies easily, no? When those companies get a promising applicant, they don't do pore thru Triplebyte failed-screens data. Even if they wanted to, I presume that information isn't actually available to them.
Cost/benefit-wise seems like it adds credibility and saves time going with Triplebyte unless there isn't 2+ YC companies you are excited to apply to.