Ah, classic whataboutism. But there are at least three things you're missing:
1. Johnson was most definitely personally involved in some of the parties, since he's been photographed at at least one of them; Starmer was not involved in the decision not to prosecute Savile;
2. Johnson's allegation was not that Starmer was merely overseeing the CPS when it failed, but also that he was personally involved (which, as we've established, is not credible, see (1) and your own link); in any case, Starmer has apologised and, once it came to light, launched an inquiry.
3. The debate yesterday was about Johnson's conduct, or about conduct in Downing Street in general. The only reason it was mentioned was as a transparent attempt to distract attention from Johnson personally, and muddy the waters.
Clearly it was a terrible failing to prosecute Savile, and clearly as head of the CPS Starmer has ultimate responsibility. But one final, obvious difference is that Starmer accepted this, whereas Johnson manifestly doesn't.