when you use model builder, look at the output of each process. the result of Add Feature Class to Topology tool is topology that has new feature class , not feature class to connect as feature layer in modelbuilder
P.S.: I also tried to connect the feature classes and topology directly to "Add Rule to Topology", it returns an error and failed...
and the error maybe because Add Rule to Topology tool couldn't find the two feature classes inside topology .
that because there is no precondition in your model.
there is two ways to solve that :
First connect the two results of the two Add Feature Class to Topology tools as precondition before Add Rule to Topology.
Or connect two Add feature Class to topology tools in Sequences not at the same time.