Hu, I was about to report the same thing.
I agree that Terzi's video does not demonstrate the issue well, but I hope the following does:
https://www.youtube.com/watch?v=JLDjxzQw2o0
As you can see, the Viper pilot in this video performs a perfect Knife Edge maneuver, which is 90° bank, rudder in counter direction to keep nose high, and this video shows quite well that the pilot is able to maintain the jet in stable attitude.
In DCS this is currently not possible. At most you can keep the nose above the horizon for about 2 seconds, but the nose actually starts to fall immediately, no matter what you do.
And like Terzi I also tried different loads, and the surprising thing is that the load does not seem to have any impact on this at all. My expectation would be that with a higher load that it was harder to keep nose high, while with empty load (no weapons, no fuel tanks, low fuel) that it was easier to keep nose high. But that's not happening in DCS, which suggests some simplification of the flight model here?
And as I was already at it, I also played with the Yaw trim knob at the left hand side of the cockpit. The surprising thing here is that it adds angle on the rudder also when rudder pedal is fully pushed. I am not sure if that is correct behaviour either. So yes, you can get some extra degrees rudder angle with Yaw trim, but still no Knife Edge with stable attitude possible in DCS. And as far as I know, real Viper pilots do not use the yaw trim for this maneuver.