So .......... (spoilers)
The aliens make mankind construct a giant machine that is so expensive to make that nation states begin to negotiate about its funding
only to send some dude or dudette on a trip so they can say "you're not alone keep reaching for the stars little buddy ok time is up, now piss off you little sucker, sending you home with no evidence where your life is ruined by my unclear instructions and garbled message"
what reason do I have to assume they aren't hostile?