I honestly don't know, but at some water depth you have to start worrying more about water intrusion inside of the vehicle. 42" is high enough that the battery pack on the Rivian would have to be wet, if not entirely submerged. (Max ground clearance is something like 15", so I'm going with "entirely submerged"). I don't think they'd be citing a 42" fording depth if it was prone to fiery battery destruction when hitting that much water. Would be a bit of a PR problem.
On a gas/diesel vehicle, if you're using a snorkel, we're talking about depths where the 12v battery is fully submerged, and apparently it doesn't short out, which is, again presumably, the same situation here.
Again, at some point, on a modern vehicle, gas/diesel/electric, you have to start worrying about floating/lack of grip, water intrusion to the cabin, filling up the HVAC system, messing with various electronic components inside the vehicle. Computers, screens, wiring harnesses, heated seats.
Just from reasoning it out, at a 42" ford depth, I don't have a reason to think it would fare worse than any other modern fossil fuel vehicle, even if said vehicle was equipped with an intake snorkel above the waterline.
I just don't get the knee-jerk assumption that somehow it's EV-ness will make it most likely worse than a gas/diesel truck, when the very thing you're doubting is (apparently) a marketing bragpoint for Rivian. Again, this is all to take their marketing material as gospel, I recognize that.