Technically, you could program your own mods in Minecraft if you had the skill. So actually that point is valid. Especially since for those without a mind for programming, Minecraft already has a little stuff prepared, and you can install other people's mods, so both games have either the same amount of options, or possibly slightly more for Minecraft, simply because of the modding community being large enough to offer more options to those that can't code.
Minecraft mods are programmed with OUTSIDE programs, not made by mojang. Roblox has programming tools build in. Also, all of Roblox is a mod, as the definition of a mod is "the state of being modified or partial alteration." All games "partially alter" roblox, so all of it is one huge mod. Finally, in Roblox, there are "free models" which allow options for people who cannot code.