You just pretty much have to ignore player power progression or the game makes no sense at all. I mean, if we take it literally, the Garlean Empire has Lv70 soldiers in a few endgame dungeons of Stormblood. I expect we'll encounter Lv80 Garleans in the new relic playfield, and possibly in the MSQ before next patch. Meanwhile the three original ARR cities continue to be defended by Lv35 troops. Presuming the Garleans are still intent on conquering all of Eorzea, they could dispatch a small task force which wouldn't even make a dent to their troops on the Lv80 front, and it would make short work of the defenders in the ARR areas. Because it makes no sense for the NPCs to remain sequestered to their level-appropriate areas, we must assume that player levels also work on a meta level and have no meaning inside the game world itself. So in-world these relic weapons are probably just different ancient weapons the protagonist collects and none of them are significantly stronger or weaker than the other.