If I understand you correctly, and you are saying that the RM must allow
assertion with only a single role in order to model such relationships,
I think you are right.
Example:
"'Hot' and 'cold' are opposites"
This would be modeled in the graph like this:
x2
/
C-----R2
/
T1                  A-----T2
|                  /
|       R1        C-----R3
|       |        /
A-------C-------x1
\
C-----R3
\
A-----T2
\
C-----R2
\
x3

The nodes above represent the following subjects:
T1: the assertion type 'at-opposites'
R1: the role 'role-opposite'
x1: the players of the role-opposite  ("The Opposites")
R2: the role 'role-setMember'
R3: the role 'role-set'
T2: the assertion type 'at-set-setMember'
x2: 'hot'
x3: 'cold'
I can't see how this could be modeled without allowing a single
membership in an assertion.