wow I had no idea. Although I personally assumed that the encasement made them waterproof (otherwise why would they mention sealing with waterproof silicon? Who cares if the seal is waterproof is the entire case isn't lol) maybe the main idea is just an added layer of protection for the epoxy finish so it keeps its protective finish (as the ad also references that it is similar to a phone case, in case you drop it.)how are you sealing your print? the prints are water soluble so encasing a magnet in PLA /PETG doesn't help.
You can put a pause in your gcode so when the printer gets to a certain spot it waits for you to put the magnets into the holes then you hit continue and the printer finishes the job.