Are the MPP fasteners rigid when the panel is pulled?

The fasteners is not rigid since it works under an elastic preload and it will move a bit if you pull the panel.

However the system will not open by just pulling the panel.

On the catalogue you can check always the tensile strength of the Turnlock quarter turn.
For the parts in the MPP series, this is 1330 N.
This means that you can pull on the with a weight of around 133kg.
This mean the Turnlock can support the pull of a mass of around 133kg.

At the same time, this is for a static load.
For a dynamic load (this would be the maximum).
There is a safety factor of 30% before the system breaks (but that can depend on the application).
Be cautious, stainless steel parts have lower tension loads.

Also, Important to consider:
Turnlocks are not designed to cope with side loads! For side loads we recommend Spanclamps.
A Turnlock quarter turn is not a product that will guarantee a certain clamp force.
not an official value!!! As it all depends on the thickness of the panels.