Wrong. This is no double standard.
#1: Djokovic is 3-2 in sets, Nadal is 2-3 in sets. Even with a 3 set win over Federer and a straight set win over Tsitsipas, Djokovic ends up with a better set record than Nadal - 5-3 vs 4-3.
#2: Nadal faces the #1 in his group on Friday, whereas Djokovic faces the #3 in his group on Thursday. Nadal already lost to the #2 and beat the #4, and Djokovic lost to the #1 and beat the #4. If Nadal beat Zverev in 3, he could lose to Tsitsipas and get through without a problem, similar to what Djokovic will do if he beats Federer. But he didn't.
#3: Groups are set fairly. Not much complaining when the draw came out because no one expected Zverev to actually be good. If someone complains about Zverev suddenly being good, they should also mention how Medvedev is suddenly choking and how Thiem is the best player in the whole WTF right about now.
#4: Everyone knew the rules heading into it. If you don't want the possibility of being eliminated, then don't lose. Simple as that. If you lose, your success is no longer in your hands - it's up to the other members of your group.
It's fair. Even if Djokovic loses tomorrow and Nadal makes it to the semifinals, it will still be fair. Nadal is in contention even by losing in straights and barely winning a 3-setter. Hell, he can even be #1 in the group if he beats Tsitsipas in 3 sets and Medvedev wins in 3. How is that fair?
Nadal could go 4-4 in sets and still be #1 in his group, while Tsitsipas could go 5-2 and end up #2. Is that fair? Yes. Because they all had the info going into the tournament. No one rigged anything. They just played and won or lost.