afaik it happened at night so it seems unlikely that optical guidance was used (unless they had added some kind of thermal sight) but more likely that guidance was slaved to the radar .
You can't really compare the U-2 incident to an F-15 - supposedly the U-2 had engine problems, was below it's normal operational altitude & was virtually at stalling speed so not exactly in the best of positions to undertake evasive action. An F-15 is far more powerful, manoeuvrable & has countermeasures.
Plus the SA-2 is supposedly quite visible - I guess that somebody got lucky/unlucky.