So, those two things (combining pitch + yaw and tri-cording) are basically the same thing.
At some point, players found that acceleration was greatest when they combined all directions at one time. This was because it maximized the number of thrusters being used. AO has a video on it: https://youtu.be/4_XVyNCiAa0
Because of this, we would have to fly at an angle while using all inputs at the same time (up/down, left/right, forward/back) to achieve the highest rate of acceleration (ie: tri-cording). Players that knew this could catch up to other players faster because it allowed them to accelerate faster. If someone is trying to run, you could tri-cord and catch them fairly easily if they didn’t know how to do it.
It was never an intended mechanic of the game, so with the new flight model changes, CIG is taking it out. The “pitch & yaw cannot be combined anymore” statement just means that using both of those directions won’t cause maneuvering thrusters to apply additional power above what the ships intended acceleration is.
E:D has a very limited turn rate to keep fights much slower. It also has a much better pitch turn rate than yaw. These changes bring SC closer to that, but it’s not really the same. SC still has much faster acceleration and turn rates.
The limit on max speed while in combat mode will force players to fight at slower speeds, but comparing it to E:D is a reach.