When in HOLD and the speed reaches the MCP speed automatically changes in THR to maintain the selected speed.
No it absolutely does not.
IF auto-throttle wake-up engages, it puts the A/T into SPD mode, and either targets a speed just outside the low speed amber band if the selected speed is INSIDE the amber band, OR targets the bugged speed if the bug is OUTSIDE the amber band.
BUT- Auto-throttle wake-up does NOT function when the A/T is in HOLD (Typically when climbing or descending in FLCH or VNAV SPD)
THAT is consistent with what you quoted here-
Note: When the pitch mode is FLCH or TOGA, or the airplane is below 400 feet above the airport on take-off, or below 100 feet radio altitude on approach, the auto-throttle will not automatically activate.
The NTSB has confirmed the A/T switches were in "ARM"- so being in FLCH and thus with A/T in "Hold" seems a likely scenario.
Yes. they would have gotten an "Airspeed Low" EICAS- but seem to have been slow to respond to it.