the lambda calculus doesn't talk about concurrency but it does discuss time. like turing machines there is a concept of time inasmuch as a particular algorithm will take so long to complete (complexity). The reason it doesnt talk about concurrency is because concurrency doesn't bring anything to the table in terms of deciding complexity or computability.

to be a little more blunt :) Talking about concurrency in the lambda calculus is like talking about concurrent math or paralelizing the concept of addition (not an implementation).