Surely not! If the bike was rolling back, the cranks would also rotate backwards. And the only time in the days when I rode fixed regularly that my cranks went backwards was when we tried - generally unsuccessfully, to ride backwards.
I'll admit it's nearly fifty years since I rode fixed, but in those days we all went fixed for the winter. We strapped ourselves in our toeclips (some foolhardy souls also used shoe plates to lock their feet in) and used our pedals as brakes, treading down on the back foot and pulling up on the front foot. It was remarkably effective, and made the bike easier to control when stopping or slowing on slippery or icy roads.
Whether that exercise strengthened my knees I don't know, but I've always ankled up hills when the going got difficult, pulling back on the backfoot from about the six o'clock to the eight o'clock position, up from eight to ten, and pressing forward from about ten. There are some hills I'd have to walk if I didn't. And there's nowt wrong with my knees, though there's other bits that don't work as well as they used to!