Spot on analytics.
Forced hand is really the only explanation other than just good old fashioned tinfoil hat conspiracy.
Alien mothership projected arrival timeframe makes sense. Even if not hostile ,the world’s religions are largely predicated on humans being alone and “in God’s image” that gets utterly crushed when a superior off world being shows up. The level of human mental meltdown will be apocalyptic all on its own.
The “alien mothership” will slowly crawl through space at sub light speed even though the alien technology allows them to open portals and appear anywhere in the universe in a matter of seconds.
That is like deciding it might be fun to travel from New York to Los Angeles by horse and covered wagon while waving at jets flying overhead.