You can just ignore it. I have been out west in the mountains where I have been above and below 4000 feet a couple times in a single day. I am sure there are others here who have done the same. Been up over 8,000 feet without and down to near sea level with the jumper connected. Never really noticed a difference.
I suspect that the MAF sensor makes a lot of the altitude adjustment since as the name implies, it is measuring the MASS of the air entering the engine and controlling the fuel injected to match. I'm thinking the plug is probably just there to lean out the mixture a little bit to reduce emissions.
