I have never heard that. I guess it makes some sense, but it seems that you would have to significantly increase the oil to lean out the mixture enough to cause a problem. If you had a finely tuned racing 2 stroke, "lean it out until it siezes, then go up one jet size", then this could be a significant problem. I don't believe the average street machine is anywhere near this state of tune. Most of them seem to jetted a little rich anyway.
Haynes manuals have a tendency to use "boiler plate" to fill out their manuals and I suspect that this may have originally been said in a different context.
Maybe Fred will chime in here with some words of wisdom.