https://mobile.twitter.com/googlewmc/status/1161725709926182...
They certainly weren't equal but they got more than a little (trust/authority/ranking - call it whatever you want) help from their parent domain.
To the point that this was the case:
If I bought a brand new, never used .com and built 500 pages of spammy content and linked those pages/content from thousands of other super spammy pages... that domain is de-indexed within 2 weeks and the pages never really ranked/got organic traffic worth the cost of the domain name.
If I built those same pages on a subdomain and linked them in the exact same manner but the parent domain was very strong - the pages would rank and traffic would flow steadily, indefinitely. (As long as the parent domains footprint/linking was large enough to absorb whatever penalty that my spammy pages brought).
It's still happening today where people buy up domains that are established with good link profiles - go to archive.org and download the sites old content, re-host it and keep it alive. Then they build spammy subdomains on the parent domain - profit.
I suspect what is more likely is that the subdomains were separate but not completely unrelated/correlated to the primary domain. Google likely assigned a certain amount of value to the mere fact that it came from the core domain. I think that is what changed here.