You are right that for most of the Thargoids' vessels, you don't need to be that fast, but there is one specific vessel, that is said to be able to reach about 530 m/s and for some missions in those Thargoid infested systems, it is quite advantageous to be able to at least keep them at distance until you can jump.
I think the 20% "rule" for disecting those generators seeped over from the experience of those "professional" Thargoid fighters who do that circle dance around them... It worked there, so it should work here, too, you know how these things roll. Better be on the safe end if you don't like to see the rebuy screen.
A single, simple sentence can easily be misunderstood as it often lacks a lot of context.