I am not sure what a "sewing machine" is or how effective Liquid Fire was, but if the plumber had to use it, then I question whether he even had the correct snake. There is almost NO WAY to diagnose whether there is a sag in the line without using a video camera. Unless the septic tank plugs up so the line can fill with water, and it has to be plugged for days, there is no way water can come up to the surface around the house where you could see it. To do that water would have to rise above its "own level" and gravity, (plus the laws of physics will not allow that to happen). I think he was either trying to "dazzle you with his brilliance", or "baffle you with his BS".