Well it doesn't actually say behind. It says 21" clearance.
I think he was being facetious? You would need 14- stacked 2x6's for blocking!
The topics covered on this site include activities in which there exists the potential for serious injury
or death. PlumbingZone.com DOES NOT guarantee the accuracy or completeness of any information contained
on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling