I spent a lot of time in the blackhat SEO world prior to 2014 or so... I can assure you that subdomains reaped a bountiful reward from their association to the main domain.
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've read that in multiple docs back then but since you observed this it could've been false. The rationale I had for it was that there are many sites with completely independent subdomains or technical resources like asset being served by subdomains. Buying established Domains and scraping old content does not rely on Subdomains. Affiliate, ads or your own product can be placed on the TLD. I dont doubt you tough and I think I will try it sometimes. Old school fun
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.