I think the problem is they knew his CV and were interviewing him for a senior role. It's a little manipulative to string him along as interviewing for a senior role when they knew up front they never would have hired him for that role.
Reading between the lines, and having been involved in tech hiring for junior - low-level senior roles, my guess is that they were open to his being at the level of a more senior role than his experience suggested, but his interview performance suggested he really was a more mid-level candidate, like you'd expect from his experience.
It is exceedingly rare that recruiting will give feedback on anything at all subjective; if the true reasoning is "your interviewing reflected your observed years of experience which puts you at mid-level" then the feedback to the candidate is going to be about their years of experience, not the interview performance.
> it wasn't based on my answers to soft questions
It seems that he didn't consider erb frontend or he dislikes it, he was expecting to work on js/react or whatever other framework he knows/likes. That demostrates that he doesn't fully understand what a frontend developer job is.