There are no such incentives.

When writing feedback, you simply look back at 20 other interviews where you asked the same question, and write "candidate X's performance on this question was was in {top,bottom} {5,25,50}% of ... at this level".

You also have no idea how other interviewers will score, and if your score is 1.5 when everyone else's is a 3.9, it's likely that the HC will ignore you.

The HCs do see multiple feedback from you and your peers, so they build a model of your feedback for themselves: P is a soft interviewer, so we'll adjust his score down but Q is a very hard interviewer so we'll adjust her score up ;-)