He does get beat though.
Look at his season:
MC- lost Evans
Belgrade- Lost Karatsev
Rome- Lost Nadal
Olympics- Lost Zverev
Lost Busta
He has won 4 tournaments all season- AO, RG, Wim. Belgrade 2.
He's only played 10 events all season, and this USO is no. 10.
The issue is slams and five sets. He has a huge advantage there. He is the most motivated. The most experienced. The most confident. His game is going to hold up better over a five set match than anyone else and he is unrivalled. Look at his stats and the rest of the USO Semi-finalists. They have nothing on him.
So, I don't think that he is harder to beat than ever. He was winning more in 2011, 2015. And it's not that he is harder to beat at slams. It's more that the field going up against him just do not have the tool box and mentality to do it. However, put some of them up against him in a best of 3 somewhere and they have a much better chance. They can sustain their best level for 2 sets much easier than 3. They are also coming up against a less motivated player. Djokovic brings it to the slams and can afford to sacrifice a higher season winning percentage to do so due to where he is at in his career. These guys still can't do that. They are much further down the career and development line.
Since they only play Bo5 (max) 4 times each year, it's an uphill battle for the contenders to get the necessary experience needed to win the big matches, going against a big3-level opponent (Novak nowadays, Rafa/Fed until recently). Winning a few early rounds count for little imo, you need experience from the hard QFs/SFs/Fs.
To win against the best retriever, neutral player and counter-attacker of all times, you first need to acknowledge a Bo5 against Novak is at best a marathon that requires a rock steady performance with as little fluctuation as possible, that will be decided in the last sprint to the finish line.
Imo, the two teens yesterday showed the type of game needed to win against such a player: you have to be constantly pressing, but just as willing to defend as attack, and ready for any type of shot. These young women managed to defend superbly through most of the match, but when in the driver's seat they also made sure to constantly keep control. The latter is much harder than it seems, bc in the men's game, it seems many try to just hit trough Novak. Since Zverev has solid groundstrokes from both wings, it seems like a plausible approach, but it backfires since he likes to stay behind the baseline and Djokovic has a great dropshot and the wheels to back it up.
If a player like Alcaraz learned to mix it up better, and open up wider angles, he would be very hard to beat, as his stock FH forces the opponent to stand way behind the baseline. But so far, he seems, just like Sinner, only interested in bashing huge groundstrokes ...
The losses Novak's suffered the last year or so, have been in B03 against offensive-playing "sprinters". That's just not possible in a Bo5 against a healthy Novak.
Otoh, a great playing Medvedev on HC is the kryptonite for any player on the ATP; he just has a way of producing awkward shots his opponents struggle to hit comfortably. Some of the passing shots he came up with against FAA were absolutely incredible and hard to fathom. If you look at his body and arm movement and rotation, it's mind boggling to me, he can place the ball exactly where he wants at times.