The article discusses the (highly speculative) hypothesis that eukaryotes arose from a virus merging with an archaeal ancestor to form a nucleus. If the hypothesis is false (it is widely believed that eukaryotes arose from a joining of archaea and bacteria, not archaea and virii) then "an archaeal ancestor" doesn't even have a referent.

The LUCA is the common ancestor of bacteria and archaea. That would have existed far earlier, as neither of those are eukaryotes.

> it is widely believed that eukaryotes arose from a joining of archaea and bacteria, not archaea and virii

IIUC the join with the bacteria is the explanation of the mitochondria (and later chloroplast). But it does not explain the nuclei that is weird too. Is it possible something like this?

  Archaeal + Virus = Pre-Eukaryote (extinct?)
  Pre-Eukaryote + Bacteria = Eukaryote (including animals, fungi and plants)
  Eukaryote + other Bacteria = Plants

I'd like to take the opportunity to post this classic about the plural of virus:

https://www.ofb.net/~jlm/virus.html

More because it's funny than that it matters.