Then why is Apple advertising it as 1000nits sustained?
If it can sustain 1000 nits for HDR content, it can for SDR too. The display isn't going to say "hey it's SDR let me piss them off" and suddenly start dissipating more heat at 1000 nits at SDR compared to HDR after all.
Well most likely because HDR content doesn’t mean full bright white for extended periods of time.
Even if you watch 8 hours of HDR content, only some of the LEDs will get to peak brightness for small periods of time. That will allow the LEDs to cool down enough to sustain that brightness.
The macOS UI and webpages on the other hand are mostly bright white and that can cause trouble on extended periods of time.
That's perfectly reasonable, but Apple lists different brightness capabilities for "sustained" and "peak". I don't think anyone expects their excellent 1600 nits peak brightness to be constantly available, but the 1000 nits sustained brightness is supposed to be, well, sustained.
Perhaps "sustained" is some kind of jargon from the laptop industry but I consider myself to be more educated than the average Joe when it comes to computers and I would've expected a 1000 nits screen based on their advertisements. I don't see what Apple has to gain by either lying or deceptively using jargon like this.
If it can sustain 1000 nits for HDR content, it can for SDR too. The display isn't going to say "hey it's SDR let me piss them off" and suddenly start dissipating more heat at 1000 nits at SDR compared to HDR after all.