That is true- grab a multimeter and measure how much resistance there is between the starter and the battery -ve. Should be less than 5ohms. If its more then you have a bad connection somewhere. I've used the starter mounting bolts as an earthwire point before, but in theory you shouldn't need to.
If there's no resistance then either starter is a dud, battery is a dud or motor is too tight. If it turns by hand then motor is not too tight, and if you used jumpers then battery supply is good, and if you've swapped starter I just don't know...