Any Disneyland Resort hotel guests can use the “Pixar Pier Guest Only” entrance and exit. I suspect they are trying to make the entrance feel more exclusive for guests of that hotel, but a key from any of the DLR hotels will open the gate.
While I think this is generally true, I had the strangest experience at this entrance/exist during our trip in late November. We were staying in DVC rooms at the Grand Californian, so that exit out of DCA is so wonderful - you can pop out of that exit and be back in your room in like a minute.
One evening, our plan had us wrapping up on Pixar Pier and then heading back to the room using that exit. We had successfully used that exit a number of times already on the same trip. That evening, there was a Cast Member standing in the "doorway" of the exit, and she asked us if we were staying at the Pixar Pier hotel. We replied, no we are at the Grand Californian, and then she wouldn't let us use the exit - said it was for Pixar Pier guest only.
It is true that there is a sign posted there that says this, but I've always been of the understanding that Grand Cal guests were welcome to use it also. Plus, we were just wanting to exit rather than enter. I don't believe in arguing with Cast Members - they work so hard to make magic for the guests - so we backed off and huddled up to discuss walking over to the other hotel exit by Grizzly River.
Well, just a couple minutes later, the Cast Member walked away, leaving the exit "unguarded," so we just exited there like we originally wanted to. It was kind of a head-scratcher.