Second response: he has an interesting point about the grammar or syntax. But that tends to become simplified over time. Who's to say that a "West Germanic" language didn't adopted a simpler grammar on its own -- or through later influence by North Germanic invaders? If English vocabulary is much closer to Dutch than to Norwegian, wouldn't that make English a West Germanic language (whatever later changes it went through)?
Be all that as it may, Scots (Lallans, not Gaelic, though there was some influence there as well) was heavily influenced by Scandinavian Vikings, as was English (only more so).
The categories are something we (in a way) create.”
Kant? Is that you? I thought you were long dead!
“whenever you have things grouped together based on similarity it’s always possible for somebody to suggest a different way if grouping them”
Not when historical causation is involved. Then matters of chronology as well as similarites intrude. English cannot be a Scandinavian language if the Ur-form of it already existed before those who spoke it came into extended contact with Scandanavians. It’s well known that both English and the various Scandanavian languages derive from German, and that Germans went en masse to England and the Scandanavians countries before Scandanavians had extended contact with the English.