According to Gubrud's situation, postseason games do count:

https://www.heraldnet.com/sports/former-ewu-qb-gubrud-wins-appeal-will-transfer-to-wsu/?

"The NCAA states that a player is eligible for a medical redshirt if he played in one-third or fewer of his team’s games. Only because the Eagles advanced to the national title game — and thus played in 15 games — did Gubrud qualify."

Gubrud's initial application was also denied, but for a different reason -- he was applying for a 6th, not 5th, year, and that requires showing 2 seasons ended due to injury in addition to satisfying the 30% rule. He appealed his denial and ended up playing for WSU in 2019.

So assuming Mercer counts as one of four games played out of a 12 game regular season, and the Aggies win their 13th game on 12/6 to play a 14th game in Fargo the following week, there seems to be a really good chance that the twins will get their medical redshirt approved. #WinForTheTwins