Answering my own question some time later after I have experimented a bit and came to the conclusion that
thickening agents do indeed work as emulsifiers, although not 100% perfect like real emulsifiers. Yet in scientific papers they are listed under emulsifiers. What happens is, that only approx. 2% of the oil separates again after emulsion
albeit they do work, the texture doesn't become as stiff as with emulsifiers. The problem is that the thickening agent made it already thick and creamy, adding oil makes it first a bit more liquid because there's simply more liquid in total. Adding more oil compensates a bit, but then it stays there. (The oil keeps emulsified to 98%)
In short, thickening agents do work as emulsifiers but are useless for emulsion, mainly because the liquid is already thick anyway and secondly the textures just doesn't become as delicious as it does in mayonnaise.
I had the feeling that starch did work a bit better than fibers like guar gum.