In this case, the employees were provided an opportunity to elect and actually made their elections; the employer merely failed to honor them. Hence, the missed opportunity is in reference to that actual election; not the maximum matched amount.
Based on the EPCRS, it would appear that you will provide the "full match", but only to the extent that the additional (full) amounts being deferred would've been entitled to the match. For instance, if someone's annual deferrals already exceeded 5% of salary prior to the missed amount, then there would be no correction for the match; since the missed deferrals wouldn't have received a match anyway.
Good Luck!