To be fair, he was born in Scotland, but during the important times he was an American.
The 1500s when he popularized kilts?
Ok quick history lesson. Kilts were a HIGHLAND warriors outfit. They were illegal for many years, and by the time people were allowed to wear them again, 9 out of 10 scots were now lowland scots. However, George, and Walter worked to popularize the kilt so that it is now worn by non-highland, non-warrior scots. Basically, the popular image of what a kilt means and of when it is acceptable to wear one comes from an American. Yes, they were worn before. But the resurgence can be narrowed down to one portrait and one man writing about them.
For a modem example: nobody wears a outercoat with a cape anymore. If Johnny Depp wore one, and then it became popular, we can credit JD with the popularity of them, right? Not the tailor who invented them in the 1850s (or whenever it was).
To talk about the americanism of sir Walter Scott, he did great stuff in scotland, England and america.
Since america > Britain though, he gets to identify as an american.
In much the same way, since America > Canada, alexander graham bell was an American and so is the telephone.
if there's one thing I can't stand, it's a butterfingered walrus!