What exactly would count as copyright infringement in a replication paper?

After asking permission to Elsevier (I'm editor in chief for ReScience), they confirmed (in this specific case) that equations can be re-used without asking permission to Elsevier:

Our Global Rights group has confirmed that these equations are not copyrighted; therefore, you can merely acknowledge the 2012 Journal of Theoretical Biology as the source of the equations but do not require written permission from Elsevier.

From what I have learned about copyright law (IANAL), the answer is non-obvious to the point that even a lawyer could only guess. Leaving aside the issue of how jurisdiction would be determined for the moment, and assuming that the US copyright law (and in particular Fair Use) applies: The problem is that the way copyright law is structured, copying the explanation of the equations would constitute copyright infringement on its face. Fair Use is merely a defense that may be applicable, and it depends on the balancing of multiple factors (the four factor test) which is by necessity somewhat subjective. The only way to truly definitively determine whether a specific act of copying is exempt by fair use or not is to have a court render judgement on this specific use.

It may be that in practice, this specific case is clear-cut enough that a lawyer could with confidence predict based on precedent if this case would be covered by fair use or not, but for that you would have to ask law.SE (in fact, you should probably ask there to make sure there aren't any errors in the above as well).