BS! All 4 link trailing arms need to move in 3 different arcs, whether IRS or solid axel. Up/down, twist, and side to side.
And if you really think about it, rubber bushings are actually less restrictive than even heim joints loaded. No friction, just a little progressive twisting of the rubber. If the bushings can't twist, then the trailing arm will try to, which is where the binding comes into play.