It doesn’t do much, and nowhere close to what actual exercise will do, but it’s better than nothing at all by a small margin.
Iirc, the last study was done back around the turn of the century. I know I was still lifting at the time, and it wasn’t long into the noughts that I had to stop doing that seriously.
And, if my memory isn’t betraying me, it gives slight increases in benefits with exercise as well. I know that visualizing things in martial arts and other “muscle memory” activities improves performance of skills over not doing it.