Differences in creme fraiche recipe
A quick search of the internets confirms your basic recipe of 2 tbs buttermilk to a cup of cream. This is likely a modern formula. Keep in mind that today's cultured buttermilk is not the same product as buttermilk (the liquid left over from churning butter...which could then be left to ferment) or "soured cream" (fermented cream) of years past.  In fact, I did find this Julia Child recipe that more closely aligns with the recipe from Le Cordon Bleu, which you reference. Julia calls for 1 cup of "soured cream" to 2 cups of heavy cream.  So, I don't think your reference has a typo.  I think it is from a time when we didn't use modern "cultured buttermilk." The bottom line is that you need some type of starter to get a lactic fermentation going.  While it might sound like too much liquid, remember that the fermentation process would thicken things up.  I would say that you could probably reproduce the traditional recipe, but it's probably more convenient (and would likely result is a very similar product) to use the smaller amount of commonly available cultured buttermilk that can be found in many markets.