I think WE are the ETs. If you look across the landscape of humanity as it continuously emerges the idea is unavoidable. Check out Michael Cremo's Forbidden Archaeology for examples of the inaccuracy of our widely accepted timeline(s). It's a long, dry read so you could just look at the very existence of Gobekli Tepe in Turkey if you don't feel like investing the time. Once you can accept that we (currently) are only another iteration vs 'The Pinnacle' of all that lies in the past it's an easier task to connect the dots.
The show had an overtly stated theory and thus an agenda to promote its hypothesis that it didn't try to hide (so it wasn't trying to come off as one of those fake documentaries the History Channel also produced), but its production values, camera work, global locations and references to actual history made it worth watching. But after a couple years, it became a bit redundant.
There are so many structural mysteries of the ancient world where we don't know how they did it - and it seems they either had methods or technologies we are not aware of and which there are no records for or remnants of. Or... they had help from something or someone much more advanced.