That is the tagline. In the phrase, "survival of the fittest," "fittest" means "most adaptable," at least according to the Merriam Webster English dictionary.
The adaptability of an organism might be an important fitness factor, or it might not. There are organisms which have survived largely unchanged for millions of years.
An adaptable organism might stay around for longer - or it might be outcompeted in the short term by less adaptable organisms.
Sure, “survival of the fittest” in the short term is obvious, but it’s hardly worth pointing out.
>or it might not
To determine that it is not, would by definition imply an observation that it is no longer fit.
In other words, how do we know that it is not adaptable? We'd see that it no longer propagates, and is therefore unfit.
Can you think of a case where these two synonyms are not inseparable in meaning as pertaining to evolution?
I can't. I think Darwin meant exactly this.