This is correct, it will take several generations across years of selective breeding to accomplish this. I know because I created a line of pure-breeding, autosexing Welbars several years ago. It took me about 2 years to get my first double barred cockerel chicks.
The black sexlinks you get in the first generation will still be nice birds, but far from autosexing.
If anyone wants to learn more about Welbars, check out my webpage:
http://welbars.com/doku.php?id=welbars:start
I have pictures of both gold and silver Welbars and links to other pages with the "recipe" to make your own. My current Welbar flocks are laying 30 to 40 eggs a day now and most are as dark as my Lavender Marans and some as dark as the best eggs from my Black Coppers. I sell a lot of their chicks to local people. Guaranteed sexable chicks are a big deal in suburban areas, as one might expect.