"The search relies on data from our crawls of the Alexa Top 1 Million sites, Search Engines, Common Crawl, Certificate Transparency, Max Mind, Team Cymru, Shoda

"The search relies on data from our crawls of the Alexa Top 1 Million sites, Search Engines, Common Crawl, Certificate Transparency, Max Mind, Team Cymru, Shodan and scans.io." So probably CT logs. Also, if you've ever sent a cold-cache query to a recursive resolver that didn't employ QNAME minimization (few do), it was likely harvested by pDNS replication at the TLD nameserver level and shared with a number of commercial and research parties' databases to which DNSDumpster may subscribe.