I have absolutely no idea of the mechanicals of it. It's just something I've read in a few different places, one such being a magazines guide on what to look out for when buying a GTI. I don't know if there has been anything proven but I just thought I'd share.
I don't want to sound like a dick and this is not aimed at you like personally or anything (
) but rather than spread information which you don't know is true (thus causing panic and worry - Dave reads your post and tells his mate who tells his mate who posts it on vwvortex and then you end up on a 500 page article where no-one actually gives any factual proof about anything and then all Mk5s are tarred with an incorrect brush come resale by "wannabe" enthusiasts making everyones lives a mes) why not do some more research into it, take the initiative ! rather than saying something mechanically makes no sense!
In THEORY running poor quality fuel could cause a lean (hot) mixture which could increase cylinder pressures and blow the headgasket. But that's just in theory
In practise the Mk5 will retard the timing and will run safely on 95 ron which is the lowest we can get here. They run 91 in America (I think they have 95 as their PREMIUM GAAAS) and I've never come across this as a problem.