In many industrial applications, the concern is mostly control of the isolated side, like because that could physically destroy stuff. Exfiltration is a smaller or nonexistent concern, since you're already sending most data out deliberately.

So there's still an attack surface, but it's a lot smaller. Any side channel exploit would need to work (at least in some initial form) without changes to the software on the isolated side, since you otherwise can't bootstrap your way to installing it.