Realistically, the dealer is probably the last place someone should seek advice about fluids.
We know transfer case part numbers changed around 2011 when Toyota started specifying the new fluid, we know it has some unique physical properties compared to other gear fluids (color, smell (indicating different additives)). But we don’t know what exactly changed within the case that led Toyota to spec the new stuff.
One very plausible explanation is 75w-90 generally being GL5 which is known to attack yellow metals which are often used in synchros.. which it seems our transfer case has.
IMO telling anyone “it’s fine” to run a non-spec fluid should have that information and the rest of the story so they can make an informed decision. Even the expensive stuff is a negligible cost per mile at the recommended change interval, to avoid any risk associated with the fluids.