The cloak has a property that causes creatures to have disadvantage on attack rolls against the one wearing it, and
This property is suppressed while you are incapacitated, restrained, or otherwise unable to move (DMG, pg 158).
The way Shell Defense is described
You can withdraw into your shell as an action… While in your shell, you are prone, your speed is 0 and can’t increase, you have disadvantage on Dexterity saving throws, you can’t take reactions, and the only action you can take is a bonus action to emerge from your shell (EGW, pg 181).
is effectively the same as being both incapacitated and restrained, with the only notable difference being that an attack against a prone creature
has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage (PHB, pg 292).
While extremely similar mechanically, the tortle using Shell Defense is technically neither restrained nor incapacitated. Furthermore, the ability to emerge from its shell as a bonus action implies the capacity to then move during that turn if the tortle chooses to, so neither are they otherwise unable to move.
As such, would the Cloak of Displacement work for a tortle using Shell Defense (and thus cancel out the advantage melee attackers would normally have due to the prone condition)?
RAW and RAI interpretations are both desired.