no it isn't an old wives tale, you can use a 12volt car battery to jump start a motorcycle but the car shoild not be running. The amperage in a running car puts out is a lot more than a motorcycle is rated for. Even some small jump boxes are rated for 2500 amps and larger.

 

The car system can supply more current but the motorcycle will only draw as much current as it needs. However if the voltage output of the car is significantly higher than the motorcycle then you are liable to cause damage.

The design of the regulator maybe such that it doesn't like a reverse bias of 14 plus volts whilst the stator output is zero.

HD state up to 15 volts is okay for the regulator output so the bike itself will be okay because the car shouldn't be putting out more than that either else it will cook its own battery in use.