Well, my earlier statement may have been a generalized WAG, sort of.
It's danged hard sometimes to remember everything you read in a manual, but I went and looked at the Komatsu publication service manuals for the K-21/25 and it essentially says that safe mode for overheat is initiated at 1,000 RPM once the ECU determines that the engine is actually overheating.
The 1,000 RPM "sound" is very close to the rated 750 RPM idle speed, so in the few cases where I have actually caught one in limp mode, I may have just considered that to be "idle speed" or even maybe a "fast idle".
The manuals further describe that a malfunction in the MIL system itself will limit engine RPM to a max of 1,500.
I did experience that once when the K-21 engine was still fairly new to us. I wondered why the engine would not run at full RPM and why if there was something wrong......the MIL was not showing a code. It turned out that the MIL did not light up during self check either and that led me to check the MIL bulb, which was defective. But that truck had been sent to us for that reason (not run right) and when I discovered the burned out MIL bulb, I also found that the cavity above the MIL bulb had been obscured with electrical tape.
I guess that somebody had gotten tired of that MIL staying lit all the time and since they lacked the ability to fix the cause, they simply "put a gag" on the messenger so the operator would not know the light was on. I guess the bulb finally failed from staying lit all the time and that was when the truck began to limp.