If you toe tap and lift your toes off the turf before landing on the sidelines, you're good for both feet down whether forward or backwards. If you try to toe tap but your heel lands on the line, it's not a toe tap, it's a step and you stepped on the line making you out of bounds. Lamb was backwards and his trailing foot actually hit the defender and deflected downward which is why the whole foot landed. So if Lamb intended to toe tap and lift his foot up before going out of bounds, the defender hitting his foot made that impossible.