Minority? Probably. Tiny? Absolutely not.
Here [0] is a breakdown of 70 popular Steam games by the source of purchase for their reviewers as of a year ago. About 28% of all Steam purchases happen outside of Steam itself, with Valve getting a 0% cut. Note that for many games a majority of reviewers did not purchase it on Steam itself.
0: https://docs.google.com/spreadsheets/u/0/d/1ICv-UE4i651yMkpD...