Summary
Using drawers, it is possible to store a shulker box inside another shulker box

Repro Steps

1. Obtain a drawer of any kind and two shulker boxes.
2. Load one shulker box (shulker box 1) with any contents. Then pick it up.
3. Place shulker box 1 inside the drawer and pick up the drawer.
4. Place the drawer inside shulker box 2.

Expected Result
Vanilla Minecraft deliberately prohibits placing shulker boxes within shulker boxes, and the above procedure does not allow this restriction to be bypassed.

Observed Result
The above procedure allows for nested shulker boxes.

This is because the drawer is not a shulker box. If drawers were modded versions of shulker boxes, then your logic would apply. But it's just a different kind of inventory that happens to support picking up with contents preserved. It is not comparable.

To fix this problem, shulker boxes would have to be modded to add drawers to the restriction if they maybe contain shulker boxes. That would be outside the scope of this mod.