The fight was mid diff. The mistake people make is that they measure the difficulty of a fight based solely on how much damage the winner took. This is the same reason people consider Pica vs Zoro low diff even though it was high diff. If you're 100x times stronger than your opponent and they can't hurt you, but their endurance and durability is so high that it takes you an entire day of straight beating to defeat them, then the fight was extreme diff even though you didn't sustain an injury.
Difficulty is simply how much did take it take in defeating your opponent. It's not "how damage did I take" but how much time and energy did it take to defeat the opponent.
Sabo doesn't have a single scratch on his body, meaning the fight was Sabo just beating on Burgess for about ten minutes. As OG Sama likes to put it, Burgess took a tour across Dressrosa. Sabo was essentially tossing him through the city in a relentless assault, and Burgess wasn't KOd because of his monstrous endurance.Ten minutes of beating down on Burgess wasn't enough to KO him, and low diff is being able to put down your opponent in a few moves. It was mid diff.