That's the entire issue with this whole argument. There are two parts to the regulations. One objective and one subjective.
The objective rule is a load test that is applied to various parts of the car such as the front and back wing. The subjective rule is that all external surfaces must remain static and immovable (whatever the hell that means ).
All cars racing now have passed the rear wing load tests and should be/are currently declared legal. Mercedes is arguing that Red Bulls rear wing visibly flexes ergo it violates the immovable surface rule.
The issue is that if you set that precedent then Mercedes are also violating this rule as their rear wing also visibly flexes.
The front wings are the same, it's hard to judge by how much but it does appear from the camera shots that mercedes front wing flexes more than the Red Bull front wing.
If Red Bull's car subsequently gets declared illegal through Mercedes court action that they are threatening to do then Red Bull will simply turn around and make their own court appeal against Mercedes front wing and the same penalty should be applied to Mercedes as the legal precedent will have been set.
This entire thing will become a tit for tat and Toto Wolf will look like a right a** hole for starting it.
The only logical way to move forward is for the FIA to change its load tests. If the the Red Bull passes the more stringent load tests but is shown to flex on camera then so be it, the car should be declared legal