I just ran a test where I had my residential zone connected to the highway and my industrial zone on a separate island and not connected to anything -- the industrial zone still developed and factories seemed mighty happy and not lacking in workers. A telecommuting paradise.
Incidentally, my residential and commercial also developed without a regional connection, as in buildings popped up within zoned areas, but they remained empty with the tooltip saying 0/whatever households. My guess is the game just checks whether anything at all is connected to the region and then starts populating the residential. Still odd to see the actual buildings pop up, albeit empty, without a regional connection.
Can't say I *really* followed development (though I pre-ordered the deluxe just to quote unquote support the devs), but I do remember CO saying they were using an agent based simulation -- the way industrial jobs get filled sure looks like a statistical model is being used, which does of course make the whole business with roads and traffic and overpasses kind of moot. Oh well.
Well, building the residential area not connected isn't that much of a problem. Just pretend it's magical movers building stuff and people not moving in (if they were moving in that would be a problem). Industrial jobs do get filled and I'm going to assume through agent based as well. It's just that the workers either teleport or work from home.
Could be just a bug or a feature that makes the game easier to play and if that's the case I'd like to see it shoved into the harder modes.