At a low level this is true. The switch isn’t electrically connected to anything, it’s supposed to trigger a switch inside the slot, and the driver or SD controller IC should suppress any writes. I’d expect the Pi’s Linux drivers to be properly behaved when encountering a read-only SD card, but the only way to know is to test it.
You need an OS designed to run from read-only storage for that to actually “work” though…I’m not familiar with any designed for this purpose that work on Pi other than OpenWRT - which might be a very viable option.
Love this project. I guess G-code would be the “modern” way to control such a robot?