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.
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.