I'd suggest that Scots descended from Northumbrian Anglo-Saxon, whereas English mainly descended from Wessex/Mercian Anglo-Saxon. When a dialect crosses over to become a language it's hard to say, but Scots certainly had an Army, so presumably a navy at the same time. Perhaps it's the case that after initial divergence, the course taken by most languages, it has undergone a reversal of its evolution and now is closer to English than it was 200-300 years ago.

So it started as a dialect of a dialect, then underwent independent development to become a language, and has now returned to being a dialect.