Those would be the temps near the entry to Hetch Hetchy. Up at Lake Eleanor, it would be a bit colder. I'm not sure exactly what trail he initially took, but I think it's this one:
That would go almost 3000 feet higher and therefore be as much as 15 or more degrees colder.
I think his plan was to do Lake Eleanor (descending a bit) and then the rangers hypothesized that he may have gone across through the Little Yosemite Valley toward Tuolumne (where he likely could have hitched a ride - Tuolumne is at 8600 feet).
There ought not to have been snow up there at the time Joel went up, but it certainly could have gotten down into the 30's overnight. He was prepared - but was he prepared (for example) for falling out of his kayak (if he made it to either lake?) This is unsigned backcountry wilderness, where only the main trail junction will (hopefully) have signs. One needs a map for sure.
It was very kind of you to pick up that couple. And you're right that there's no signage - which is clearly stated in Yosemite's wilderness information. When one gets a wilderness permit, they remind you again. The trails that start from Tuolumne and from Lake Tenaya get far more use, but there are still missing people up there - some very perplexing cases that must surely be wilderness misadventure. Way easier to find one's way in those areas - the area Joel went into apparently attracts a few day hikers (going by trail forums) but if he did something like attempt to get to Tuolumne, there would have been a lot of trail junctions, possibly unsigned and some of them so-called "social trails," meaning that humans have made trails that the park doesn't mark as real trails and never signs.
