It looks like this loophole is not explicitly discussed in the "axioms", but it is mentioned in the paragraph before equation (2) which I copy here:

A symmetry transformation is said to be an internal
symmetry transformation if it **commutes** with P. This
implies that it acts only on particle-type indices, and
has no matrix elements between particles of different
four-momentum or different spin. A group composed
of such transformations is called an internal symmetry
group.

By demanding that commutators vanish, they exclude SUSY, since not all the supercharges commute with all Poincare generators. See for example equation (2.53) in [1].

A very nice discussion regarding the Coleman Mandula theorem can also be found in [2].

Hope this helps!

**References**

[1]http://people.sissa.it/~bertmat/lect2.pdf

[2]Weinberg’s QFT book,Vol. III, chapter 24.B

This post imported from StackExchange Physics at 2014-03-31 16:04 (UCT), posted by SE-user Heterotic