What makes you say that Google claims it's not true? Google claims subdomains are completely two different domains and you'll lose all the linking/page rank stuff according to their own docs regarding SEO. Some SEO gurus claim it's not so black and white but no one knows for sure. The data does show having docs on subdomain is more harmful to your SEO if you get linked to then a lot.

Here's the argument for/against it: https://www.searchenginejournal.com/ranking-factors/subdomai...

I think the answer likely is quite nuanced, for what it's worth.