Which is a long way of saying, yes, I agree with you; the methylation metric may be total bunk. But there isn't anything else (IIRC) that we can all agree to measure instead. That's why the Harvard article emphasized that they were using a metric for aging they'd developed in-house. I just want to read the paper before passing judgement, and right now sci hub doesn't have it.
The experiment was to test whether vitamin D supplements could attenuate behavioral problems associated with fetal alcohol syndrome. Our lab used a rat model of FAS. When the post doc told me about her experiment idea, I (as an undergrad at the time) told her it was interesting but couldn't be performed in a rat model because vitamin D was poison to rats. It calcifies their skin because they cant break D down like humans can. My warning was ignored, and the experiment was conducted anyway. She found that vitamin D treatment mitigated hyperactivity associated with FAS, and presented this finding at conferences. Well no shit this treatment would reduce activity in rats. If my skin was calcified I wouldn't want to move either.