Pretty sure Steve was using the Gitzo 4 series monopod, sure he will confirm when its daylight Stateside , I'm using the Gitzo with either a D5 or D500 attached to the 300mm f/2.8 and 500mm f/4 with no problems at all.
Also using the Kirk foot on both lenses with no issues.
Sorry, editing this post. Yes, I am using a 4 series Gitzo monopod