Sorry, folks, this one doesn’t flunk the aggie test.
First of all, at a cruising altitude of 60K (or higher), I’m surprised the U-2 would even be in the ATC system. The odds of another airplane colliding with a U-2 are approximately zero, since most jetliners have a service ceiling of no more than 45K. In fact, I find it surprising the U-2 would have its transponder on; if you’re the only aircraft at that FL, no reason to squawk, and no reason to switch it back on until you’re descending back into Beale, or whatever the final destination was.
Secondly, U-2s transit through SoCal airspace all the time, so the handling of this type of track shouldn’t confuse the ATC system (again, assuming the Dragon Lady is being tracked at 60K.
I’m wondering if the U-2 explanation is actually a cover for a test flight of the purported spy plane that was recently sighted over Texas. It would also certainly be a high-altitude platform; there are plenty of aviation firms in the Palmdale area (where the incident occurred) and Edwards AFB isn’t far away.
As a side note, when you are getting ready to drop down into more crowded airspace, it is good to have been talking with them so you coordinated your decent with ATC. . .just so you don't surprise them.