Luanti and Minecraft are two distinct, if similar-looking things.

Luanti is an open-source voxel game engine implementation which allows running a wide variety of different ‘games’ on it (including two which mimic Minecraft very closely).

Minecraft is the closed-source game owned by Mojang.

The two don’t interact and servers for the one are completely unrelated to the other as well.

So, to answer the question - yes, they still need a Minecraft license if they want to play Minecraft. But this is disconnected from having a Luanti server, for which you don’t need any licenses but which will in turn only allow you to also play Luanti stuff, not Minecraft.