That sounds like a feedback email from a couple years ago - am I right about that? We've definitely gotten better at this over time.
The bigger difference that I see is your statement that "Every few months, I check that all our recommended resources are still up-to-date, available, and free."
In some unfruitful further communication with TripleByte, I asked how I was supposed to improve, given that their feedback seemed to be "we have no qualms with your ability, but we don't think you can pass an interview". They recommended that I sign up on interviewing.io. I did that, and a year later, I was still on the wait list. (I eventually got off the waitlist when complaining about this same sequence of events on HN, and an interviewing.io employee saw the complaint. I don't think that approach scales well.)
It's hard to take this at face value, as 100% of the TripleByte messaging I've seen since before I received that email focused heavily on how confident they were that their interviews were thorough, high-quality assessments.
Edit: You also need to consider that TripleByte's idea of what works is probably different from yours. Their idea is probably more along the lines of "people pass our assessment and get hired." All you really need to do to hit that (not that it's a trivial thing at all), is conduct an assessment that's similar enough to what the hiring companies are doing. And, many of us know that hiring companies frequently aren't very good at interviewing.