They model the above as dh(t)/dt, generalizing the discrete case (the equations you wrote) to a continuous case. Check the eq 2 in the paper. The statement fol

They model the above as dh(t)/dt, generalizing the discrete case (the equations you wrote) to a continuous case. Check the eq 2 in the paper. The statement following the equation makes it clear that "Starting from the input layer h(0), we can define the output layer h(T) to be the solution to this ODE initial value problem at some time T". Here, as per my understanding, h(0) can be the input itself. The function f mentioned in the eq 2 is the RNN cell.