“This is, of course, a very unrealistic assumption … these operations are often extremely costly … to prevent many transactions.”
That was him referring to the oft-straw man argument by opponents of Coasean insights about “transactions costs being zero.” Coase obviously knew that. And he obviously was making a different point. The Coase Theorem as you are told about it is not the Coase Theorem as is. In any case, Coase did not coin the term Coase Theorem either.