I can understand why someone would say open source
I can understand why too: it’s either because they were not aware of the widely agreed-upon definition of the term, or because they’re being disingenuous. I’m assuming it was the former; whether OP edits the post will reveal if it was actually the latter.
It isn’t about the list of approved licenses, it’s about the criteria for being added to the list. New licenses regularly meet the definition. This license clearly does not.