Yes, what you said is exactly true. The plate material definitely affects flexibility. A PC plate will have greater flex than a steel plate. If you want greater flex, it is actually better to opt for a half plate or plateless build for maximum flex, but of course, you are sacrificing the stability of switches and sound. Of course, the mounting style also affects the flex. A gasket-mounted keyboard typically has greater flex than a traditional tray-mount keyboard. However, the plate material definitely affects it more.

Edit: Of course, I want to note that these assumptions are based upon the sources here: Ways to manipulate sound profile of keyboard: https://www.youtube.com/watch?v=_N8k0u75eLQ&t=603s&ab_channel=Keybored

Methods determining flex: https://www.keyboard.university/200-courses/plate-materials-sizes-9yn46

https://www.keyboard.university/200-courses/keyboard-mounting-styles-4lpp7