We had silver paint at IBM 50 years ago. There is nothing new there. I would question how much current it will carry tho. We only used it to repair CCROS cards. (Printed circuits on a paper IBM card used as ROM) It was milliamps if not micro amps.
I doubt this will ever be a practical way to run line voltage and low voltage would require a lot of current.
Used to be able to buy something like this to repair damage to printed-on elements of car rear window heaters. If it worked, (I never tried it), it must have taken several amps.
This stuff has been around for ages as "silver print", used for repairing damaged PC board traces, old time alarm window foil, etc. Also as mentioned a repair material for auto defroster grids.
I question the statement that it can used on metal. How would that make a circuit. The entire metal plate would short everything out. It would need a coating of sort not bare metal.
'Marmite' is a brand of ..er.. sandwich spread [?], made from hydrolysed yeast- a by product of brewing beer. Very high vitamin B content. Powerful stuff, very salty and an aquired British taste, although half the pop. can't stand it! Also sold in New Zealand and OZ as a slightly different formula.