What would you guys consider a "link" exactly? Anything short of merged PNRs? "link" implies a connection between two things while those things remain distinct. It's a perfectly cromulent word to use here.
Since we're going there, CarmenOM made clear that she was using link in a colloquial way, not to imply the PNRs were merged or the same. That should satisfy the purists who, correctly, maintain that PNRs can be referenced, which is useful in IRROPs to keep a party together, but the technical term is not link. Those purists would say that the idea of linking the PNRs would suggest that they go together automatically, that changing one would change the other, and that may be a correct interpretation. However, there are many reasons a party may have more than one PNR, and Delta recognizes that - for example in the fact that you can upgrade a companion on another PNR. Finally, certainly regular FTers in this forum understand the difference between one PNR and PNRs that reference each other.