[andrewflnr]

> you, me, or anyone else alive

One of these is not like the others.

Why do you think it's less plausible that humans have this technology than any other intelligent entity? There's no real reason to believe there's a sharp line between "human" and "other" capabilities. If the alternative is literally aliens, it's not a large stretch to imagine that once you've had the right insight, a Mach effect drive or something similar is possible.

The part of that story that's harder to believe is that we could have successfully kept it hidden this long.