To know why Tor Project itself doesn’t speak on this matter, you’d need to wait for a reply from somebody from the project.
Are you aware of any articles from torproject or research papers confirming that hosting tor relay at your own IP does in fact help your own traffic blend in?
Specifically for Tor? No. For exactly the same reason I can’t point you to any research that confirms, that downloading 500 kB/s and 200 kB/s over Tor requires 700 kB/s. It’s a trivial consequence of basic knowledge for the given field.
You are right, I might have been going too deep into weeds and missed that this was already addressed on tor relay operators FAQs: https://support.torproject.org/relay-operators/#relay-operators_better-anony...
It confirms that running a relays helps operators own traffic to blend in and mitigate some attacks. It may seem trivial now, but I've learned not to make assumptions with higher risk projects like Tor.