In his own words: "Finally, I take full responsibility for the technical decision to develop the GNU kernel based on Mach, a decision which seems to have been responsible for the slowness of the development. I thought using Mach would speed the work by saving us a large part of the job, but I was wrong."
That's not him saying using a microkernel was a mistake, it's him saying that he was mistaken about how long it would take, and the impact Mach would have on development time.
http://www.linuxtoday.com/infrastructure/2000101200421OPCYRH