I think I get it lol
Enter at your own risk.
Sorry I didn't have much of an answer for you earlier, because honestly I don't know what the sign really means, though I do believe it is what you think. I've been down it numerous times, along with official rigs I've seen on this forum, and nobody has been bothered.
When GPS originally came out for cellphones and cars, it used to take people this way into Yosemite if you were coming from the Highway 49 areas of California. Needless to say it's a one lane road, small cliffs at times, so people would be calling 911. With that said, I do believe what you think, it's a warning that this isn't a regular road, even though it should be clear as the roads leading up to that point are nothing different.
Besides a few rutts here and there, and a rocky river bed looking section, the worst thing you'll have is mud after rain. It is closed in the winter times however due to deep snow.
It's a fun little route I like to take from now and then to look for wildlife. If my research is right, it was originally a Native American Indian trail at one time, which then was turned into a road to get around the tolls back in the late 1800's or early 1900's. I think the man who owned the Wawona area didn't want his guest to have to pay a fee.