I kinda glossed over the interview structure for the sake of brevity, but they do speak to one technical person (me) before the tech assessment. I'm the hiring manager, and I see my primary role as getting the candidate excited for the position and to answer any initial questions. I've already narrowed down the applicant pool through resume review at that stage, so most people make it to the technical assessment from there. I feel like technical assessment shouldn't come too much later in the process: we're trying to be respectful of time on both sides, and while the assessment is 2 hours long, it's only completed once by the candidate, whereas if we have the technical interviewer come earlier on in the process, it will add many more hours across candidates who would have otherwise been filtered out.
As far as using 3rd party solutions: I see your point, and actually used to administer homespun "code challenges". The downside with that approach is that it adds a ton of overhead. As terrible as it may seem, we may not be able to invest that kind of time unless we're willing to be more heavy-handed in candidate selection early on. In at least one case, there was a candidate who had a fairly unremarkable resume but really impressed us in the interview process, and they may have been eliminated earlier on if we were more selective from the beginning.
Late last year we had a candidate who seemed really promising: she had a CS degree from an Ivy League school and had a few years of work experience using relevant technologies.
She absolutely killed the Byteboard technical assessment, and seemed like a good cultural fit during my initial screen. I advised the technical interviewer that he didn't need to go too deep; I was satisfied with her answers on the technical assessment. Then something odd happened - she completely bombed the interview. The interviewer told me that she couldn't even answer the most basic questions e.g. "what tech stack does your employer use?" I was thoroughly confused about that, but still passed her on to the VPE to help us get to the bottom of the discrepancy. Our VPE confirmed the previous technical interviewer's assessment.
We came to the conclusion that she could not have completed the technical assessment on her own and obviously didn't move forward from there.