Lucasfilm published a Star Wars newsletter called Bantha Tracks and the 5th issue, published in July 1979, says:

“Not much is known about Boba Fett. He wears part of the uniform of the Imperial Shocktroopers, warriors from the olden time. Shocktroopers came from the far side of the galaxy and there aren’t many of them left. They were wiped out by the Jedi Knights during the Clone Wars. Whether he was a shocktrooper or not is unknown. He is the best bounty hunter in the galaxy, and cares little for whom he works — as long as they pay.”

I believe that this is what the ESB novelization and the Marvel comic were going by.

The Empire Strikes Back Sketchbook, a book about Joe Johnston’s concept art for ESB, says that that the initial idea was for the armor of Boba Fett to be worn by “supercommandos” “from the Mandalore system”, hence the Marvel comic introducing the planet Mandalore and naming the warriors in Fett-like armor “Mandalorian Protectors”.