Average is a neutral PSxG of zero. Given that De Gea is a goalkeeper who adds nothing whatsoever in terms of most things goalkeepers are generally good at such as dealing with crosses, managing the game, sweeping, organising defenders in front of him, I'm sure you'll agree that you'd expect his shot stopping statistics to be significantly to the positive side of the spectrum? 18/19 minus 1.1, 19/20 minus 0.3, 20/21 minus 1.9, 21/22 plus 1.1, 22/23 minus 2.2. He's generally one of the worst two or three goalkeepers in the league for the other measurables such as sweeping and dealing with crosses. So yeah, considerably below average.
I didn't say the save was luck, no. But I wouldn't expect a Premier League goalkeeper to let a header like that in unless they were miles out of position, which he wasn't. He's positioned well and probably could even hold it but he very rarely holds anything so it's not a surprise he didn't hold it and I'm not going to criticise him for it; in reality if it goes in major questions are being asked.