I basically agree with you here, even if as a long practicing Systems Engineering specialist myself, I would say Systems Theory is what you used to identify the edges of the box imposed artificially by the "closed box thinkers", so they could apply SLT to it, if I interpret what you wrote correctly.
The problem seems to be that those imposing the box have gone beyond realising they imposed a box, by diving into the detailed implications, the more invested they are in the diving, the more difficult it is to move back out far enough to see the box.
Systems thinking is out of the box thinking by definition.
You are practicing Systems thinking by identifying the box, it seems to me, so I think it is a little unjust to identify systems theory as the limiter, it is actually the opposite, imho.
Notice also there are some who will claim to be systems thinkers whilst demonstrating clearly they are not, this can also cause some confusion :)