I suspect Morodiene means the joint; having hand in line with the forearm, and the whole forearm are hand above the level of the keys. Then there will be room to get the thumb under.
Some scale practice tips - there are many variations on these themes for endless hours of fun
Take one scale:
Play it as you do now, at your best max speed with all the humps and bumps. Record the tempo (metronome!)
From now work one hand at a time (HS)
Play scale slowly and velvety, legato, slow enough that you are playing evenly. LH then RH.
Play at a medium tempo quietly with a distinct accent every 3rd note. Keep going up and down the scale until the accent arrives on the final note. Don't use 3 octaves or you only get to go up and down once! Use 4 or 2 octaves. LH then RH. This section can be extended to other groups of accents (e.g. 5s, 3 + 2, 6s, 7s...) but do this exercise only once if this sort of practice is new. Listen to your hands and don't overdo it.
Play forte, LH then RH, just once. Omit this one if hands are tired.
Play piano, LH, RH, Hands Together (at last) focus on evenness and legato and calm hands
Another to try: piano, crescendo to top, decrescendo down again
With these last 2, choose medium, fast and slow tempi.
Then do your test run 2H, listen to evenness, and check your tempo. Hopefully you'll notice some definite improvement even after one session.
Let me know if this helps, hope I've pitched it at a useful level