That doesn't work as Joplin scales the page up when the screen is rotated (Previous post below). So you see the same "width" of text regardless of the rotation. At least it does for me on Lineage 16 and Google Android 9. (Joplin 1.0.316)
Edit: Cancel the above. Scales correctly with 1.0.318. Will just have to wait for the Play Store version to catch up...