10% of the top million sites are dead 4 years ago
As near as I can tell, these are the top 1,000,000 domains referred to by other websites they crawled.
The report is described as "The million domains we find with the most referring subnets"[1] and a referring subnet is a host with a webpage which points at the domain.
So to the grandparent, presumably if something is "linking" to these domains, they probably were meant to be websites.
[1] https://majestic.com/reports/majestic-million [2] https://majestic.com/help/glossary#RefSubnets, https://majestic.com/help/glossary#RefIPs and also https://majestic.com/help/glossary#Csubnet