Is an ethernet cable threaded inside a metal conduit is more protected from electromagnetic interference?
You discovered wrong. Ethernet runs at 100MHz and up, your line voltage at 60Hz is virtually DC at that frequently difference.
With that said, you should still run your data cables in conduit. Not for interference concerns, but so that in 2040 you can easily replace the cat5 with something new for our 20K ultra VR direct brain video or whatever is the deal then. Plastic Smurf tube is fine, it doesn't need to be metal.