Meta Front End Engineer interview questions

A recruiter approached me through email. I had a quick online interview with typical HR questions (experience, why would you like to join, etc) and 4 very simple technical questions. The interviewer was very polite and explained the entire process. Afterwards, I received a coding challenge via email and was given 3 weeks to complete it. The interviewer garanteed I could take my time. I completed it in something like 10 hours over the span of 1 week and a half (busy week at work) and delivered the results pretty confident that I wrote clean code, with tooling, comments and even specs. After that, in half a day I received a single "no" with zero feedback. I suspect they are not measuring your ability to solve the problem, but something else, as it was pretty much solved perfectly... Anyways, I was saddened by the lack of feedback - as others pointed out, if we gave them hours of time, they could at least point what was missing.