Nope....Muscle Shoals named after the "Muscle Shoals Rythym Section "....which was not a band....but 3 studio musicians...who basically only worked in the studio...even though they did give touring a try after being talked into it by Steve Winwood to join 'Traffic' on the road for a short while.
Muscle Shoals recorded over 50 albums in one year during their heyday....and anyone who was anyone went there to record in the 70's.
Leon Russell is credited for giving them the nickname "Swampers"....but at the time Ronnie Van Zant wrote the lyrics, they were still going by "Muscle Shoals Rythym Section "....Since Ronnie died in '77... why he did it can only be answered by conjecture, ....(one theory is it didn't fit the timing) ....so, therefore, unanswerable....
However, it's generally conceded that although Russell coined the nickname, the group only adopted it after 'Sweet Home Alabama' became a hit.
Ummmm...."the day the music died " refers to the plane crash that killed Buddy Holly.....et. all....
EDIT: As studio musicians, they were recorded on almost EVERY hit record in the 70's!