They can. What they're asking is if you're willing to allow them the key knowing they have the base game, but no dlc. If you say you don't, then you can reroll anyone with the base game but no dlc. If you say they can, then anyone who doesn't have the dlc but the base game can join and win with your permission

GA creators have full control over whether or not entrants are allowed to receive their wins, if that entrant already owns part of the content of the win. So, if they own the base game and enter for a Complete Edition giveaway, it's up to the GA creator to decide whether or not the entrant is allowed to claim the win or not (though it's polite for the GA creator to state their preferences in the GA description, to avoid any unnecessary stresses on entrants and staff).

This doesn't hold true if the [GOTY, etc] version is considered different by way of Steam- for example, you can enter for Deponia: The Complete Journey despite owning all three games seperately, as that's considered a different game listing within Steam (with minor differences in content compared to the individual games). Likewise, if there's no way to upgrade to the content (say, if the GOTY is the only listing still remaining on Steam, and all DLC/upgrade options have been removed) then staff may also not allow the reroll.

But normally for subs like this one, where they list the individual components out, you do have full rights to a reroll in any circumstance where a user owns any of the components.

Allowing users to enter for DLC can be an uncertain thing, as they may already own a lot of the DLC and just not be mentioning it. To check DLC ownership, add the user on Steam and then check the store page for each DLC- it'll list the user among the "Friends who already own this game" section if they already own the DLC in question.