Well I can dig that the Empire got there before the falcon ..But getting the trajectory from straight after leaving the junk again it's a ??? Initially a bit far fetched. And there is no evidence that Fett got to Bespin before the Falcon it is ambiguous.

Wether or not you trust wookiepedia I found this so you can make of it what you will.

Before the rise of the Galactic Empire, cloaking devices were virtually unknown but not unheard of. Around 32 BBY, Sienar Design Systems developed and built a cloaking field generator for Darth Maul's Scimitar, powered by stygium crystals. Boba Fett also had one, which had been added onto Slave I by Mentis Qinx, a dockyard owner on Tatooine, in 22 BBY.

If you choose to Hang on Needahs words fine... But there seems to be plenty of contradictory claims of the technology and that it's plausible he was not up to date of current technological advancement.