Apparently the SU-27s have a ceiling of 62,000 feet.
Pretty impressive but they would have precious little maneuvering capability at that altitude
I am pretty sure that a SU-27 pilot is not going for a collision with the U-2. It's much easier to launch a missile which doesn't care much about the altitude. China licensed and manufactures an ancient (1983) R-27 that has the range of about 100 miles. This missile performs poorly in a dogfight, but will easily take a sitting duck like U-2 - especially if Chinese improved the design with modern components and algorithms.
“Apparently the SU-27s have a ceiling of 62,000 feet.”
And the solid fueled air-to-air missile makes up the rest. An 80,000’ intercept is quite feasible.