Steam will check your account when you attempt to upload a mod if you have a licence/access to the game. If you do not, the upload will fail.
As the steam workshop handles mods for ARK, it is required to own the game.
Making mods for ARK does not require the purchase of the game, but there will be no real way for you to do anything with the mod.
Also there’s no real reason to not buy the game, it’s $30 USD so it won’t exactly break the bank, and it’s a AAA-grade game to a certain extent for almost half the price usually.