It seems right, on many MB forums it is said that, like:
https://www.benzworld.org/threads/long-wheelbase-or-not.1516195/
https://mbworld.org/forums/s-class-w220/918090-recommissioning-my-2006-swb-s350.html

Also wikipedia mentions that, and comparing dimensions (also with other years) from this guide it's confirmed:
https://www.edmunds.com/mercedes-benz/s-class/2006/review/
https://www.edmunds.com/mercedes-benz/s-class/2006/st-100529121/features-specs/
https://www.edmunds.com/mercedes-benz/s-class/2006/st-100521806/features-specs/

This is for USA, here it is mentioned that Canada could have gotten SWB W221 too but it is unconfirmed: https://www.reddit.com/r/mercedes_benz/comments/1dikawu/how_hard_it_is_to_find_a- _nonl_s_class/