Without getting into the nature of Glorantha after the Hero Wars, it is worth remembering that twice before there have been widespread catastophes that are generally considered to have ended the Age. These catastrophes were generally followed by “Dark Ages” during which knowledge and understanding of the past events were lost. As a result, we view previous Ages is if “through a glass, darkly.”
The First Age is considered to have ended with the final battle of Arkat and Gbaji in 450 ST, although in Ralios it is said to end with apotheosis of Arkat in 500 ST. Those few political entities that survived the Gbaji Wars (basically Seshnela) had changed. A few others suppressed during the Gbaji Wars (Dara Happa, Shadowlands) were restored in some version of another, others were restored by Arkat but did not long survive him (Kingdom of the Heortlings, etc.).
It took more than a century for many of the lands affected by the Gbaji Wars to recover. By then a kaleidoscope of new tribes and kingdoms had sprouted up. The later Empire of the Wyrms Friends recalled the Broken Council about as well as Homer did the chronicles of the Hittite Kings or the records of Mycenae.
Similarly, when the Second Age collapsed with two centuries of catastrophes – the Closing, the Sundering of Seshnela and Slontos, and then the Dragonkill War – old empires failed, learning was lost, and mutual mistrust ruled. Carmania, ruled by usurping Bull worshipers, was the new power in Peloria as all others were broken by the Dragonkill War. Kethaela was a splintered backwater, Seshnela gone, etc. It is more than a century before the Lunar Empire rises, two centuries before Belintar arrives on the shores of his Holy Country.
So even though the Red Moon “falls” in 1655, and Argrath is apotheosised in 1658, that does not begin the new Dark Age. If you look at the “true” dates given in King of Sartar, Inkarne remains Queen of Dragon Pass until 1743 (if you trust that), and the Illiteracy Era does not begin until around 1825. Written records reappear around 1965 and the next century or so is another revival of written sources.
Which as an aside, is pretty similar to how things happened after the Gbaji Wars and the Closing / Sundering / Dragonkill War.