Macross II is not an alternate universe, it's an in-universe production made in the aftermath of Space War I. It's NUN propaganda designed as an argument to convince the general population to embrace the colonization initiative, especially after setbacks like the SDF-2 went missing in deep space. It portrays a natural outcome of what would happen if Earth decided to turtle in the Sol System and not go out and explore and colonize. Better to face the horrors of deep space out there proactively than waiting like sitting ducks on Earth just to get annihilated again. That also explains why there are M2 references peppered through later in the franchise as well like M7's Jamming Birds - because it was a popular mini-series among those boomers, the way Fire Bomber is popular with the boomers in Frontier like Ozma.