Yes, it is, however these graphics raws aren't shipped in Classic, in classic instead of directories full of raws, you get an almost empty folder with an info.txt file that says something to the effect of "for save compatibility".
I'm not asking him to distribute the art, because that's against his contract, but at least have graphics RAWs in there so making a compatible graphic pack is as simple as making proper graphics.
//edit: This is what I mean on the example of "data\vanilla\vanilla_creatures_graphics\graphics" folder which in premium is:
https://i.imgur.com/WDn0s6z.png\
And in the classic (winrar screenshot because I can't be bothered to unpack classic):
https://i.imgur.com/k4HiEfR.pngWell - there's not even a "graphics" subfolder in vanilla_creatures_graphics which would contain the raw. And the info.txt is:
https://i.imgur.com/torgNgL.pngI understand why "images" folder would be either missing, empty, or contained empty images, but the raws should be there so custom tilesets that work both in Classic and Premium could be developed.