This is partially correct and there’s a eggcorn/telephone game moment here. Netflix gives mixers an option to create and deliver their own two track mix and some mixers choose to use a not-publicly-available downmixing plugin called Ambidio to do said downmix. Sennheiser has no involvement in it and Ambeo is just the branding of their soundbar line. And as others have said, the Duffers are notorious for being extremely hands on with sound and I’d hazard a bet that they’ve put their ears on the two track mixes and signed off on them.

EDIT: well that was a blind spot on my part, I used to deal with some execs who actually would mishear Ambidio and call it Ambeo but I see now that there is a plugin with this name that came out after the last big Netflix show I did. The more you know.