How can they know that they found "all the PERVs"? (porcine endogenous retroviruses)
You are right, they should have written "all currently known PERVs".
Although the structure of ERVs is generally well understood (https://www.ncbi.nlm.nih.gov/pubmed/26104559; https://www.ncbi.nlm.nih.gov/pubmed/26818261) there may be some complex cases that have been missed. Also, the map of the porcine genome is not fully complete yet (nor the human one) so you cannot be sure that there are no ERV-derived sequences hidden in the unsequenced regions.