It is badly worded in the FAQ, but I don't think it matters so long as you receive 60 rollover nights when you have stayed >120 nights (as Diamond) the previous year.

You can only rollover nights into the year immediately after the year in which you accumulated them.

So any rollover nights >60 would have no value; you've already hit the diamond qualification threshold for the following year with 60 nights.

Rollover nights don't count towards anything else other than re-qualification so >60 would have no practical purpose.