That's my understanding too. Dex is for an acute emergency, not prevention.
Although they do recommend that it be reserved for treatment, according to the CDC Dexamethasone is effective both for prevention and treatment of AMS, HACE, and even HAPE:
From
https://wwwnc.cdc.gov/travel/yellowbook/2018/the-pre-travel-consultation/altitude-illnessDEXAMETHASONE
Dexamethasone is effective for preventing and treating AMS and HACE and prevents HAPE as well. Unlike acetazolamide, if the drug is discontinued at altitude before acclimatization, mild rebound can occur. Acetazolamide is preferable to prevent AMS while ascending, with dexamethasone reserved for treatment, as an adjunct to descent. The adult dose is 4 mg every 6 hours. An increasing trend is to use dexamethasone for “summit day” on high peaks such as Kilimanjaro and Aconcagua, in order to prevent abrupt altitude illness.
