I know what you're thinking, which is that surely brake-stalling the trans has an element of friction or slippage, like slipping the clutch in a manual, but it's not the same as a neutral drop, and to explain why, you have to see what an auto transmission looks like inside.
Here's a trans I had rebuild a while ago; it was an electronic 4spd auto, the sort that was quite common until the early 2000s. The 8, 9 and 10 spd autos we have today are ironically more compact, but the principles are the same, and it's far easier to visualise how it all works with an older style trans like this one.
The way an auto works is that each gear is like a spinning drum (which you can see several in the pic above). And here's one below:
In Neutral, all 4 drums are spinning, and no drive is being sent to the wheels. In the pic above, you can just about see the clutch band wrap around the drum.
When you select Drive, what happens is that hydraulic pressure wraps the band tight around the drum, forcing it to stop spinning. This engages gears inside the drum, and 1st gear is now selected. When the trans shifts to second, the hydraulics release the 1st gear band, allowing the 1st gear drum to spin again, and engages the 2nd gear band which stops the 2nd gear drum from spinning, and so on and so forth.
But unlike the clutch in a manual, the clutch band in an auto is a frail looking thing, with only a very thin friction lining. It's immersed in a bath of transmission oil, and it engages with hydraulic pressure, rather than the massive clamping force of a manual clutch spring. Here's some clutch bands below, and as you can see they're not terribly robust and are really just strong enough to disengage one gear and select the next. They aren't actually designed to transmit the torque of the engine at all.
When you do a neutral drop, you have the 1st gear drum spinning at huuuge rpm (instead of idle rpm like normal when you shift from P to D) and the little clutch band has to arrest it and engage 1st gear. Basically it isn't designed for any of that, and isn't strong enough to absorb the shock from a clutch dump (or neutral drop). And that's why a neutral drop is a very damaging thing to do to an auto trans.
So now you're thinking...so what happens when you brake-stall it to 1500rpm? Well now you know that when you're doing a brake-stall launch, the selector is in D, the 1st gear drum is locked in by the clutch band and the trans is good to go.
The "slippage" is not inside the trans at all, but in the torque convertor, which looks like this:
Best way to understand what it does, is to imagine that the donut-shaped casing is bolted to the flywheel, and spins at engine rpm. Now imagine a shaft from the trans going into the donut, attached to say, a paddlewheel from an old style steamship. Now imagine that the inside surface of the donut is full of vanes, and the whole shebang is filled with custard.
As you rev the engine, the donut casing will spin, and the vanes will move the custard, which will eventually move the paddlewheel...and the transmission will now spin, too. Inside the torque convertor the metal parts don't contact each other at all, and it's the custard that transmits the force from outer to inner.
Now, the torque convertor is always working with a certain amount of slippage, for example when you're stationary in D...the donut is spinning at idle rpm but the paddlewheel is held stationary by your foot on the brake. This happens all day, everyday. The transmission fluid (it's not actually custard in there 🙂 ) is designed to cope with that. Brake-stalling increases the amount of slippage, but it isn't actually a mechanical slippage or impact; it's basically just the custard/fluid that is being whipped into more of a frenzy than usual.
Now, if you brake-stalled it to 1500rpm and held it there for a really long time, then you can imagine that the fluid will start to overheat, and that's not good for it. But a brief brake-stall for a heartbeat as part of a launch is actually far less of a mechanical shock than dropping the clutch in a manual. A neutral drop is actually very similar to a clutch dump in a manual, except that the components taking up all the shock aren't really designed for it.