Why mixing hot syrup (sugar + water) into yolks in sabayon? Recently I browsed for tiramisu recipes where ricotta replaces the mascarpone.

In this particular (italian) recipe the sabayon is started by adding hot syrup to whipped yolks: I have always mixed crystal sugar to yolks. 

What is the point of using hot syrup instead of sugar?
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.