I would not worry about it. Years ago most furnaces started in high fire to get a good flame established and then dropped to low after 30 seconds. That is not uncommon practice. My sister's new 2 stage Rheem is programmed for that. Low fire pressure switches are set very low and are sensitive to venting etc. Lennox just wants to start the furnace is a safe condition with enough draft/venting and once it fires/warms up the draft changes anyway so it is fine. Newer furnaces are very sensitive to draft and to get 95% and higher efficiency and the small physical size of the heat exchanger compromises have to be made. Check the venting outside for obstructions. I would not try start a furnace on low fire as the gas pressure/manifold pressure is so low that getting a good stable flame quickly is hard but I suspect they are trying to do that to make the furnace quieter for advertising purposes. No problem starting it on high fire.