The point isn't to seal the ribbon itself. The point is to avoid having to break the camera seal to allow the ribbon to pass through it. Why doesn't anyone get this? It's really simple and strait forward. You cannot seal a camera if you slice open the seal to pass a ribbon cable through it.
You seem to be thinking of "weather sealing" as some sort of complete, impervious barrier through which nothing passes. That is simply not the case. The sealing comprises a set of O-rings, gaskets, and in some cases foam or tape, that are applied around holes in the camera or lens shell. External switches have wires which pass through the sealing without compromising its integrity. O-rings are called that because they have a hole in the middle…they aren't a membrane over an opening, they allow something to pass through that hole and into the camera interior. The same type of sealing could be done with a ribbon cable.
The difference with o-rings & gaskets and the like is they are not at wearable joints. They are put in place, and they stay in place, and there is no wear on them. The most worn seals in a DSLR are the seals around the buttons, underneath which is a full seal that separates the button itself from the electronics inside.
My point is that an articulating screen is something that will constantly be pulled out and put back, on a continual basis, over the life of the camera. The joint there is going to take a lot of wear (especially if it's one of those swivel joints.)
In fact, it has been done with a ribbon cable in a fully articulating joint – the Olympus E-3 and E-5 are weather-sealed dSLRs with fully articulated screens...and those cameras are durable and robustly built.
I am aware that it's been done. I would also be willing to bet that Olympus has encased the ribbon itself in the seal, rather than breaking the seal to allow the ribbon through.