IMHO this is one of those times where you have to decide whether you’re willing to have a fight with the dealer to get the service which they legally must provide under law ... I’d probably do what they suggest, run two tanks of 98 through it, then when that doesn’t fix it (because in my opinion it won’t) take it back & complain.
Just a few thoughts ...
When it stalls, does it happen immediately? Or does it stutter & grumble & sputter & then stop? Because if it’s spluttering then MAYBE running fuel with cleaners (ie. PULP98) through it might clean out an injector, but I think that’s very unlikely.
If it’s just light-switch stalling, my guess is that you’ve got a flaky engine sensor. In my (admittedly not that extensive) experience, the most likely cause would be a crank-angle sensor being naughty; and to detect that problem in our 1995 Commodore the tech had to be plugged into the ECU with the laptop when it happened, it didn’t log the sensor problem for some reason.
If you don’t want to fight with the dealer & just want it fixed now, taking the car to a qualified mechanic does NOT void any statutory warranty. If you have a warranty with the dealer that’s beyond the required-by-law 3 month warranty, and they’re already not honouring it by fixing this, then if it were me I’d happily take it to a mechanic I trusted as that warranty is worth little.