How does one misinterpret a pilot reporting "Left FL X/ X000'" with a heading instruction?
Because of lazy RT.
A response of
"Left 200" meaning "leaving fl 200" is easily interpreted
as "im turning left heading 200".
Something like this is EXACTLY why the incident occurred and hence
emphasis on correct RT and use of "leaving" was highlighted.
Two different words is impossible to confuse.
You often hear people drop the "flight level" part . If you use "left" instead of "leaving" it is easy to see how confusion can occur .