Why mixing hot syrup (sugar + water) into yolks in sabayon?
For sabayon, you have to cook the eggs to hold the foam. Normally, you first mix air into the (cold) sugar and yolk, then heat on a water bath. This looks like an alternative method for heating the yolks, similar to the Italian method for meringue. 

I haven't used it, so cannot tell you how much it differs. If it behaves like Italian meringue, it will probably produce a slightly more stable foam, but with less volume, and will taste sweeter.