Synchromesh has the viscosity of an atf, but without all the additives and frictions modifiers that are present in modern atf. ATf is a neat fluid for it's application, as it needs to be a lubricant, but it can't lubricate so well that the clutches slip when they're not supposed to. I've also read modern atf has things in it that will corrode/eat brass, which makes it a no-no in some transmissions, which is why oems created manual trans fluids.
ZF wanted the light viscosity for their transmissions, which is why they spec'd atf over gear oil, which was the norm in manual transmissions. Gear oils will withstand high pressures better then atf, but if it can't get into tight spaces to lube stuff, it's not going to work as a lube. ATF being thinner will get into tight spots better. MTF will do the same. I put mtf in a lot of manual transmissions and it works very well. I also put it in transfercases too, with no ill effects. In my eyes it's essentially a synthetic low viscosity atf without the friction modifiers for clutches.