Currently all BI DLCs are defines as required. This is confusing.
Only APEX and Contact are required - and only if Tanoa and Enoch/Linovia are currently played.

The BI DLC is made on purpose to be able to play even if one doesn't own the DLC - only the terrains are exceptions.

If clothing/weapons/vehicles should be considered too, the mission would need to set a flag (dynamically or by the mission designer).
That flag would than be transmitted via steam protocol and thus the Server browser in Arma 3 launcher could actually make a specific check for DLC requirements and player ownership.

Currently this is a bad user experience and misleading. It appears rather to be about promoting users buying all BI DLCs.