I know a few of the people there, and I can vouch for the fact that they are extremely thoughtful about what makes for good learning. It shows in the product. Rather than putting lectures and videos front and center, problems and puzzles take precedence, all presented in a structured progression.
His Recommendations page, "Other places online to explore math", a companion to his book recommendations post. The page carries a footnote: "Some of the companies listed above have sponsored 3Blue1Brown videos in the past, but the recommendations here are not sponsored or advertisements in any way." He does not say which companies, so every commercial learning platform on the page is labelled as a disclosed relationship.
2020-06-01·3blue1brown.com ↗