It is non trivial to identify parked domains, so I wouldn't trust that number. Especially since their linked source itself states something else.
Is there a DNS entry at all? Is the server returning 200/400/500? Could it just be a temporary error? Maybe there's an auth wall? Or the site is used behind some corporate firewall or VPN? What if it's 200 with a "buy this page" response? Or some spam site entirely?
This. I have a handful of domains, most of which are effectively unused at the moment -- but I have them point at one of my other live sites. They're "parked", but I don't think anyone could know that without asking me.
Is there a DNS entry at all? Is the server returning 200/400/500? Could it just be a temporary error? Maybe there's an auth wall? Or the site is used behind some corporate firewall or VPN? What if it's 200 with a "buy this page" response? Or some spam site entirely?