It's the wrong definition. Strictly speaking, the term "Deep Space Nine Relaunch" referred only to the initial promotional push at the beginning of the post-finale DS9 series. The word "launch," of course, means to start or initiate something -- in this case, in the sense of a product launch, the product being the book series, which was being started up again, i.e. relaunched, with a new direction. But by association, fans came to apply the term inaccurately to the entire post-finale series, and then to other post-finale series as well.
One could say that usage is at least figuratively applicable, since these book series were all restarted with new directions. But literally or figuratively, the term is a reference to the books themselves as publications and products, and to the editorial direction behind them, rather than to in-universe chronology. Yes, for TNG, DS9, and VGR, and I suppose for ENT, the "relaunches" were post-finale series, but that's because those respective book series all had books published during the run of those series and therefore restricted in their storytelling freedom and prone to being contradicted and overwritten. So having the opportunity to tell stories set after the series, unburdened by their continuity, definitely constituted a fresh start and allowed a fresh approach. It's the difference between standalone books that didn't really count and interconnected books that do count, insofar as they affect each other.
But TOS isn't like that, since aside from Mission to Horatius, all of its original novels were published after the series had ended. Not to mention that TOS never had a series finale in the same sense as the others, and has enormous gaps of time between various movies. So if Pocket did want to relaunch the TOS novel line -- i.e. begin a new dedicated series of books with their own distinct focus, continuity, and approach -- it would make no sense to base it on the same kind of during/after dichotomy that applies to the other series. It could easily go in the gap between TMP and TWOK, or between TFF and TUC.