I think Octo Expansion isn't entirely to draw causals in, like the story campaign. You learn how to use the weapons, beat the final boss, and go into online being a better player. I think the original was so that someone who wasn't so good at shooters might still be a possible customer.
Octo Expansion, on the other hand, will hopefully give something new to the not in the sense so they can attract more players, but rather keep their community alive and going. I mean, most people I know who also play Splatoon 2 rushed out to buy the DLC as soon as possible, so it's working.
'
Basic Translation: most of it is hard to understand, but I can just figure out two things the tweet says: It's not very common to see passengers down there, and that the people on these underground subways look very different than the people above ground. this means we'll be seeing many species that exist other than Inklings and Octarians.