Mostly myth.
You can make some connections in the private practice world doing a residency or fellowship in California (mostly if you are at a community hospital or a program which rotates with private practice physicians rather than academics) but it doesn't make a significant difference in the job hunt.
Private practice is obviously dependent in many cases (depending on specialty) on referrals. I suppose you can make the case that the more people you meet when you are in training, the more referrals you might get once you start practice, but I'll bet its only a very small number. Most of your referrals will come from your marketing skills, and through the practice you join (presuming you aren't starting your own practice), as well as word of mouth once you start practicing.
Most people I know who took jobs in California trained elsewhere and wanted to be back home; the rest of us Californians were smart and decided on practice elsewhere because the pay is so much better than in Cali.