Mate its not illogical as head gasket has passages for the passage of different liquids in it, water being the least viscous of them all. It means if there is a leak, water is most likely to escape through it which we wouldn't generally notice as it would evaporate the time it gets out because of the heat of the engine, before we could get out of the car, open the bonnet and take a look at it. So technically adding this kind of a liquid, which is a sealant, to the water would make it pass through the escape route towards outside the head of the engine and then seal it while idling.
I haven't tried it yet I tried to put a bit of logic in it