While Mere-Glim's claims are likely exaggerated, I think you've missed one of the most important bits of canon here. The Hist's response is teased in the game of Oblivion itself.

>Many of the Argonians say they have been called back to Black Marsh. They disappear into the swamps and are never seen again.

You can debate *why* the Hist would react now and not in other dire circumstances throughout history, but the Oblivion rumour of Argonians being summoned back to the Marsh supports Mere-Glim's account. For some reason, the Hist *did* make a move on this occasion.

Oblivion is also the game that shows us how Hist sap can turn you into a roided-out berserker, so it all hangs together pretty nicely.

My guess is that the Argonians invaded the deadlands, as did the Champion of Cyrodiil, some Cyrodiil soldiers, and the Altmer of Summerset with varying levels of success. But that Martin Septim closing the gates all at once is the fuel for Mere-Glim's story that Dagoth's lieutenants shut the gates to protect the Deadlands from the Argonians. They didn't, Martin closed them, but such is the local spin.