This is supremely ignorant. Scots is its own language. It's a 'brother' or 'sister' of English, with both English and Scots being descendants of West Germanic languages.
The fact that many (all?) Scots speakers also speak English doesn't mean Scots not a language on its own.
You could make your exact same arguments that Irish isn't a language because you could ask a Cork taxi driver whether he knows any English.
Scots = a language with some of the same ancestors as English.
Scottish English = a dialect (and accent) of English
Scots Gaelic = another language, with the same ancestors as Irish and Manx.