My accounts were created separately in the early days of YouTube and they got linked against my will. It doesn’t show up as my gmail account in YouTube because I kept rejecting their “please merge” requests, but besides the display name, they’re essentially merged and I have to log in with my gmail account. I don’t use my gmail for anything else anymore so I don’t care, but it was frustrating back when it happened. I know it’s just anecdotal and not a proper reference like you requested..
Oh, but you're talking about a pre-Google YouTube account being merged with a Google account, not two Google accounts being linked, right? Those are pretty different things.