nah ... case dismissed.
The bass notes descending chromatically from a minor chord is a very common noodle when picking a guitar - and it features in a lot of classical music.
The tempo is very similar and you've also got the flute playing long notes over the top of it - but this was the era of hippy/stoner folk so would have been a fairly obvious arrangment to go for.
I thinks its entirely possible that jimmy page and co wrote Stairway having never heard the spirit track