Hey,
The post-match summary is a bit misleading in this case.
The Overkillmonger medal requires 80 final blows without dying, while the summary shows defeats (kills + assists). In that particular match you only had 68 final blows, so a few short of what's needed for the medal:
https://destinytracker.com/destiny-2/pgcr/10749293840
English
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Aw, that stinks but makes sense. Thanks for the clarification! <3