I think the meaning is the following: Function composition usually operates at non-collection type inputs. If you compose f and g, that usually means f takes s

I think the meaning is the following: Function composition usually operates at non-collection type inputs. If you compose f and g, that usually means f takes some input and then g takes as input the output of g. Transducers are a generalization of composition in the sense that it takes a collection of those inputs f would accepts. So, in that sense, function composition could be a specific instance of transducers with number of inputs = 1.