In the case where a regular redshirt was used, even the old medical redshirt would not have been available. It would violate the 5 years to play 4 rule. That is where you could request a 6th year of eligibility, which is much tougher to get. Some thought a "medical redshirt" was like a 2nd redshirt, but you were only allowed 1 in total, didn't matter which you used.
I think the confusion came where some called the 6th year of eligibility a medical redshirt. It's separate and is pretty strict. 2/3ds vote from a committee, extenuating circumstances, you have to miss more than just 1 season, etc.
The old medical redshirt was very straight forward. You played (ineligible for standard RS), but played no more than 30% of games (makes sense why they chose 4 games for new rule), injured/ill, and have your RS available, you could use it here.
Some of this is now outdated, but I always liked this guy's explanation:
Redshirts, Medical Redshirts, and 6th Years