Mr. Harding is a well respected maker of oil paints. I do not think he would suggest using MDF board if he did not consider it stable. His answer came as the result of my query as to what he thought was the most stable surface to work on today. I was decrying thje lack of properly prepared lead canvas on the market. He had no reason monetary or otherwise to suggest it to me other than he thought it was the most archival.
Here is another point of view on the subject:
http://www.hudsonhighland.com/hardboardMDF.htm