I was wondering a similar thing - even as simple as wear-and-tear on your joints, hips and back is less as a smaller bodyweight. I notice I even look younger at a lower bodyweight (up to a point)
Slowing in this context means going down. Basically they look at 'age acceleration', I.e. how old are you epigentically compared to chronologically. They saw a reduction of several years in this measure over a much shorter period, basically meaning their epigenetic ages went down.
Although one of the clocks they used, DunedinPACE, only looks at pace of ageing, so in that case you can only infer that it slowed (as you do not get an 'epigenetic age' figure from DunedinPACE).