I feel like people miss the entire point of the joke, which is that while obviously the pokemon would win, you take the lions' side and come up with increasingly ridiculous ways for them to win. Kyogre floods the globe? Lion raft. Deoxys shoots lasers from space? Lion ladder. Lions win.
You only need a single Mewtwo to fight a billion lions. Putting Dialga and Palkia in would be cheating, and having Arceus would be literally blasphemy.
But having Pyroar in the team... well
Now we have a mating session. 1 billion sex ( ͡° ͜ʖ ͡°)
incomprehensible to me how anyone could think the pokemon even stand a chance against a billion lions. one lion, scary enough. now imagine a lion, but there are a billion of them. what could possibly win against that? simple as