I think you are taking the meaning of multiple inheritance in a wrong way. Probably what you have in mind as multiple inheritance is like this:
class A inherits class B
class B inherits class C
If so, then that is wrong. That is not what multiple inheritance is, and Ruby has no problem with that. What multiple inheritance really means is this:
class A inherits class B
class A inherits class C
And you surely cannot do this in Ruby.