Because it is
the same version of the Cell Games that this version of Vegeta has already experienced.
"This guy is you in the past" is direct confirmation that this is the SAME Vegeta, just at different points in time.
And again,
why does Vegeta being a novice matter if he'd have immeasurable speed anyways? If you need some special experience in order to use your immeasurable speed properly, then nobody without that experience has any business scaling to immeasurable (or at least it wouldn't be combat applicable lol).
Just make up your mind on whether or not experience is needed to fly through time. If you do not need experience, then Vegeta's thing is a cut and dry anti-feat. If you do need experience, then nobody but experienced time patrollers should scale to immeasurable.