Bold: Baseless.
How do you know Burgess isn't far behind Sabo in regards to strength? Sabo was clearly superior to Burgess in the fight. Also Vergo's after shocks with a Haki imbued Bamboo stick could bend metal walls.
1. Sabo showed concern about Burgess' strength even before starting fighting him
2. He barely broke through Burgess's haki and physically it remained in a standstil
3. Burgess was pressuring him too much that he actually had no time to protect Rebecca against Diamante(that he promised to protect) and had to ask Bartolomeo to take care of that in his place. And both were clashing meanwhile with a great smile on their face as if they were both enjoying the fight
4. Sabo had to break the platform(priorly weakened by Burgess' attacks without him knowing) to steal the fruit instead of continuing the fight. Sabo just showed to be smarter than Burgess by using his strength against him, taking advantage of the diversion created by destroying the platform to steal the fruit
Yes Sabo is stronger but Burgess wasn't portrayed to be far from him