are modeling paints already thinned though? at least from personal experience. a lot of people paint with regular art acrylics (me included, cause those damn modeling paints are ridiculously overpriced imo and you can get matte medium if shiny is not your thing) and you definitely need to thin those. but modeling paints are damn near airbrush consistency - might as well be inks. thinning them further to get a transparent wash that settles into crevices effect - yes. otherwise - unnecessary.
usualy when you see those "thin your paints" memes, its about people using regular paints.