I'd agree that it's in a non-technical way. Most people only think of "the endangered species list". Colloquially, "Not on" would be on Least Concern (which is every species that's not on anything higher), and anything higher than (including Vulnerable) that would be "on the list". The paragraph your parent quotes further explains the details as to what specifically was happened, which also educates people that what they may believe is a binary state (on/off the list) is actually more granular than that.
Edit to add: looks like there's a step between Least Concern and Vulnerable: Near-threatened. Also, the definition of Vulnerable is "faces a high risk of endangerment in the medium term."
