Honestly, I'm far from an expert in goalkeeping so I won't lie to you and say I can explain it very well. It's just something I've read from many goalkeeping coaches.
Keepers often have less time to react to shots at the near post, and split seconds are crucial in that position. If they completely close off the near post and get beaten at the far post, people don't usually criticize them even if they might not be covering the optimal amount of goal. Obviously, it's far from a perfect rule and should be considered on a case to case basis, I just think fans automatically assign blame to GKs in those situations because it's something that's repeated by pundits all the time. For example, I think blaming Ryan on that Greenwood goal is incredibly harsh. Greenwood unbalances him with the stepovers and he can shoot anywhere, but manages to sneak the ball through Ryan's legs. Maybe a top class keeper does better but that's such a hard shot to get down to and there's a reason Greenwood keeps scoring like that. The second was a deflection as you say which is incredibly hard to judge, but I agree that with how close to it Ryan got he could have reacted better.
Here's a Tifo video about that goal De Gea conceded vs Crystal Palace early in the season. The title is a bit misleading as it unfortunately doesn't really go into why the near post is a myth but it's a nice analysis of why positioning and split second decisions are so important to the position.
You should read that Kepa article though, it's excellent and utilizes video examples well to get the point across.
The whole point of guarding your near post is to force the attacker to take a shot across the goal i.e the harder chance. Obviously, it happens and the attacker will casually open up their body and put it in the far post but it's playing to these percentages that makes elite positioning/keeping. Yes it's a very general rule and repeated a lot but it's there for a reason just like 'don't let the ball bounce from a high ball'. It's pretty much the basics and fundamentals of football.
As for Ryan's involvement in the goals, you have to ask if he was already placed at his near post and well positioned, why did he get beat there?
For Greenwood's goal, his feet was planted. Considering Greenwood was in motion (i.e with momentum), if he shot at the near post or far post, it would have been hit with some power. To have your feet planted, he was essentially ball watching and not preparing himself for the several possible outcomes. For Bruno's goal, he stutter stepped to his right and does okay to react but should have got there in all honesty.
Put it this way, I would have been disappointed with De Gea if he let those in.
When I have some time I'll read the article and video but I can't get my head around anybody even professional keepers saying they shouldn't be prioritising their near post. Usually those chances are presented in a way that the near post is the 'only' reasonable shot an attacker can take. So more times than not, the keeper should be protecting that post; conversely unless it's an absolute rocket of a shot, pin point accuracy, bit of good skill, deviation, gave the keeper the eyes etc situation, then any other scenario i.e just a a well struck shot
should be a high percentage save for the keeper.
I can think of many examples of poor keeping at the near post. Rashford against Spurs. Martial against Ederson (away and home!), Henderson recently against Newcastle and Arsenal etc.
P.S this is deja vu but I'm pretty sure I've debated you or someone about this before (probably in De Gea's thread)!