The French and Norwegians seemed to think so during the 16th. Century. Add in the the fact that it has a lot more "true" English vocabulary left over from Old English and some deviation in grammar from "Queen's English", I'd say it's as least as much a language apart as Afrikaans is from Dutch (despite what "popular opinion" has to say about it). Hell, you could make the argument that the Scandinavian languages are all the same if all we're going off of is inter-intelligibility and/or popular usage.
Getting to the OP, perhaps Northumbria becomes England's "unifier" instead of Wessex? There is a dialect continuum between that part of England and the Scottish Lowlands, so even though it wouldn't technically be "Scots" maybe a near-relation becomes the standard of "English" through the years? Even with a Norman invasion (*SPIT*) still happening, it would result in a different tongue compared to OTL.
EDIT: And then there is
THIS to back it up as well.