I don't see how it's "zealously anti-capitalist" to suggest that people with advantages should use them in a way that enlarges the pie, rather than primarily working to redirect part of it to oneself. The theory behind free market capitalism is that the best way to enlarge your portion of the pie tends to align with the best way to enlarge the whole pie, so it's actually a pro-capitalist suggestion.

Whether or not Bill Gates did this is another question entirely.