Not true. You just need a park admission ticket, which you can buy at the GC concierge desk, which is what we did the first day in 2015. They were more than happy to take our money, and once we had a ticket, any other time we went in from that entrance it worked like any maingate ticket entrance: they scanned the ticket and let us in. Just happened to be from the Grand Californian. The lobby and hallways of it are open to walk-in guests like any other hotel.
Driving in and parking there would be a different story, but walk-ins were welcome... which brings me to something else: Be sure to visit the Grand Californian and the original Disneyland Hotel while you are within walking distance. Both are nice diversions from the parks and are beautifully themed. The DL Hotel itself has a unique and wonderful history. And it also has a cool monorail-themed pool, along with the original tiki bar poolside. Any WDW resorts veteran will also recognize WDW's Wilderness Lodge design elements in the Great Californian, which came after it.