Scots and English: another view

Is Scots readily understandable by and English person and vice versa in the 17th century or not? I assume M. Pittock overstates the case. But constructing a narrative such as our textbook is a difficult architecture on shifting sands.

Pittock, Murray G. H. Inventing and Resisting Britain: Cultural Identities in Britain and Ireland, 1685-1789. New York: St. Martin's Press, 1997, p. 45: “It would perhaps be not too far off the mark to suggest that about a quarter of the Scottish population spoke Gaelic in 1700 (the vast bulk of the rest speaking Scots, a tongue (now attenuated almost to extinction) as separate from English as Dutch is from German). Wales and Ireland were very heavily Celtic-speaking at the same date.”