output impedance seen by the inductor
He's finding the Thevenin resistance across the inductor. It's a two port network and the circuit shown in the figure is equivalent circuit of it. It's equivalent ckt will have lower port of input and output grounded. See here for all type of equivalent circuits. Since capacitor is getting open circuited and i/p voltage source short circuited we are getting Thevenin resistance as R1 + R2.