If you were on Jupiter measuring Voyager’s speed before and after it interacted with Jupiter, you wouldn’t measure a net effect. It’s only if you’re standing on the Sun (or somewhere else where you can see Jupiter revolving) that you see Jupiter “pull” the spacecraft along, thereby imparting velocity.

Gravity doesn’t do any work. The gravitational potential energy of the Voyager-Jovian system is entirely conserved in a flyby. Jupiter’s orbital energy about the Sun is what’s stolen.

This is a common misconception when it comes to gravity assists. It’s why I think that language could be tighter.