There are loads of continuity errors, but
that isn't one of them.
The warp drive scale was 'adjusted' sometime in the decades prior to the TNG era. In TOS era used to be the cube of the warp factor was the number of times the speed of light you were moving. In the TNG era it's 3.3 instead of a flat 3, which already skews it so that Warp 9 in the new scale is much faster than Warp 9 in the old scale. But they also added some sort of 'open ended' asymptotic curve for speeds over Warp 9 so you really couldn't ever reach Warp 10.
But it's true that the Trek universe is a hopeless mess as far as internal consistency of science and technology. Paramount actually writes scripts based on story ideas and then asks technical 'consultants' make up the technology issues to fit the story!
