It is BS. A cut down string will have a larger diameter where it enters the tuning post so on a small diameter post there's a chance it might be prone to slipping out of tune more often. Still, that wouldn't affect the tone a bit.
The only other downside of cutting a longer string down is that the first time you wind it around the post, there's always a chance of breaking the string or separating the core from the winding. But if it survives that first time, you're okay. I never had that happen when I had short scale basses, it's happened to others occasionally. Probably also depends on the brand of string and how it's made.