The icon (e.g Apex icon) in Arsenal does not mean that that you need Apex to use this content, only that it was added with Apex.
A confusion can follow and it does not help creating "Apex-free" missions.

The Arma 3 DLC Restrictions page lists the issue well:
"Worse", the weapon launch_MRAWS_olive_rail_F has Tanks DLC icon, but is usable by base-game only owners.
Headgear H_Tank_black_F can be used, while U_Tank_green_F cannot - and there is no in-game notice about it.

As for the request:
An easy fix would be to add an icon next to the DLC icon to all Premium assets (e.g a star ⭐).
A "costlier" version would be to add a closed lock icon (🔒) to the non-owned Premium asset, and an open lock icon (🔓) to the owned ones.

See also: getAssetDLCInfo