Darwin coined the term 'natural selection' because had made an analogy with 'artificial selection' as done by breeders, an analogy Wallace hadn't made when he developed his version of the theory. The phrase 'survival of the fittest' was originally due to Herbert Spencer some years before the Origin . According to Popper, any situation where species exist is compatible with Darwinian explanation, because if those species were not adapted, they would not exist. That is, Popper says, we define adaptation as that which is sufficient for existence in a given environment. Therefore, since nothing is ruled out, the theory has no explanatory power, for everything is ruled in.
Darwinian theory rules out quite a lot.
It rules out the existence of inefficient organisms when more efficient organisms are about. It rules out new species being established without ancestral species. All of these hypotheses are more or less testable, and conform to the standards of science. The answer to this version of the argument is the same as to the simplistic version - adaptation is not just defined in terms of what survives. Adaptation is a functional notion, not a logical or semantic a priori definition, despite what Popper thought.