Presumably Lemaître is a credible expert, but either way, the writer’s argument doesn’t rely on his credibility or the credibility of any other expert. Instead, she addresses the accuracy of two theories’ predictions and the inadequacy of Lemaître's theory.

This is the cookie-cutter flaw of equivocation, where the argument uses the same term in two different ways. The writer doesn’t make this mistake; all of her terms are used clearly and consistently throughout her argument.

The writer doesn’t assume that one observed phenomenon caused another. In fact, she only addresses one observed phenomenon in the first place— galaxies accelerating away from each other.

In other words, the fact that the oscillating universe theory also predicted the observed data doesn't prove that Lemaître's theory is inadequate.

The writer never assumes that Lemaître's theory and the oscillating universe theory are the only explanations for why galaxies are accelerating away from each other. Instead, she assumes that because both theories predicted this phenomenon, Lemaître's theory must be inadequate.