If we assume that Is is right at the choke feed to the amplifier, and that the 28v source is a true power supply, with zero ohms of AC reactance, then the current flowing at that point will be a steady state direct current, equal to whatever average current is flowing through the transistors. The choke is an inductor, it has reactance. This allows a circuit to have DC on one side, and AC + DC on the other. In a DC circuit, the instantaneous current flow is equal to the average current flow over many cycles, assuming the transmitter is operating at the same power output. The inductor (choke) has some inductive reactance at the frequency of operation. We don't know the value of inductor, and we don't know the frequency. But it really doesn't matter if we assume that it was properly designed for optimum results. From a DC perspective, the amplifier will look like a resistive load, not like a pulsating current load. A scope placed across the DC feed will show this to be true.