It isn't perfect but there are shortcuts to change the focus to the note list/body/sidebar (default ctrl+shift+L/B/S) and once there you can use the arrow keys to navigate up and down
The current system works but requires 3 steps: Switch the focus, navigate with arrows, switch the focus again.
Using Ctrl + PgUp/PgDn can accomplish this in one passage, plus it is a system that many users already know from other software (Firefox and Notepad++ just to give some examples, use these shortcuts to navigate between tabs)