Yes, it is USD. I got the email recently and went to check it out. Their FAQ says no tax on Starpoints purchases though.
Didnt seem worth it to me.
Since you used your SPG card to purchase the points, you'll get dinged for an additional 2.5% in FX. It will probably work out to around $650 CAD for 20K points, or 3.2+ cents per point, which is likely above most redemption values. Definitely will be some places where it seems worth it.