"Issue" is probably the wrong word. Technical design feature of the Kappa engine is direct injection of fuel into the cylinder similar to a diesel engine. This means no flow of fuel over the intake valves and thus no cleaning benefit to the intake valves. As I read the propaganda online from Chevron, this would be the major benefit of Techron. Thus even less benefit to us versus normal port injected cars.
Techron (and other additives) can reduce deposits on valves in a few thousand miles, so your brother is generally right. However it is not possible for additives to clean the intake valves on the Niro. I'm not sure what that means for valve issues in the Niro, but most of those valve deposits come from gasoline that is no longer flowing directly over and through the intake valves.
Car manuals tend to use boilerplate relevant to the majority of a manufacturer's cars, and like the internet, they often propagate and continue old discredited "wisdom". For example, in some markets, they have ludicrously short oil change intervals. In the case of Britain, there does seem to be a quality difference between "supermarket" gas and branded gas (possibly just water contamination). Thus, a blanket recommendation for more expensive gas that seems to be in their manual for all countries. Doesn't cost them anything to add a few words of warning that may actually be worthwhile in some areas.
Again, completely anecdotal, but I've never had an engine problem that could be traced to deposits. I invariably buy the cheapest possible gas, but in this country, that often means I'm buying from branded stations as they have to be competitive. A Marathon dealer in my home town went independent about 6 months ago, but he tells me the exact same company still fills his tanks with the exact same gas. My PT Cruiser warped a head from overheating at about 100,000 miles (a common issue to that engine I read) and I had a look at it at the dealer when it was apart. Spotless valves and cylinders!
2018 Kia LX HEV Metal Stream with Advanced Tech