Sinkholing is a DNS action. URL filtering works on http/https connections.
When you try to visit a website, you have to resolve the FQDN (www.google.com) first. So DNS query happens first. Only after that a client (browser) knows where to send traffic (a http connection) to.
So whitelisting a URL doesn't help because the DNS query was blocked before there was any http/https connection.
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