This isn’t compelling. The use of “cookies” isn’t a problem. Just about every single web site on the internet uses them. A given “cookie” is “per domain” and is only accessible by the domain (server) which created it. It isn’t some generally searchable data point by every other web site.
It would require additional back-end collaboration between the two entities - requiring some shared knowledge of the user, like knowing an email address. While this is done by many big-tech companies, there’s no evidence here that this involves duckduckgo.com
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