Is there any public statement or other public source for that statement? In particular, it would be very interesting to read about the concrete reasons which lead him to this conclusion.
Sorry, I got that from private communication with Thomas Bushnell, who was the principle architect on the project for a long time. His claim is that they had two choices, go with the microkernel, or start with the BSD code that was legally in the clear and rewrite the parts that were under a legal cloud at the time.
Thomas preferred the latter, Stallman the former. As events proved, the BSD approach would have been fine (particularly since the legal issues eventually got cleared up), while the microkernel approach ran into much larger unexpected roadblocks than anticipated.
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.
