That's the way I would read it. The ASLRB states (8.31) that units must use all SW "up to the unit's normal operation capabilities", which I would read as being two SW's. So the unit's inherent FP is forfeited for the remainder of the turn, though the MG's can continue to fire in FPF. I suppose that, in theory, both SW's could end up malfed (especially since Sustained Fire lowers the B#, though I'm not sure that's in SK) and the unit would be defenseless. Seems pretty odd to me, but that's what the rules appear to say.