>Is this sufficiently extraordinary? https://www.youtube.com/watch?v=ZBtMbBPzqHY

I have no idea. I'm not going to watch some random video. Where is the associated published, peer-reviewed paper?

In the absence of hard data to support whatever the above purports to show, it's just as believable as this[0].

>Also, it's already conceptually possible to get around the spacetime problem: https://en.wikipedia.org/wiki/Alcubierre_drive (This doesn't imply such a thing would be easy to build; but at least it's theoretically possible, maybe)

Is it? AIUI, the Alcubierre "drive" (it's not a propulsion system per se, but rather a mechanism for manipulating space-time, but that's something of a quibble) requires "negative mass/energy." From the Wikipedia article you linked:

...a spacecraft could achieve apparent faster-
than-light travel if a configurable energy-
density field lower than that of vacuum (that is,
negative mass) could be created.

While the math of General Relativity allows for the creation of such a "warp" field, there isn't any (known) mechanism (or the means for generating the energy concentration required) for creating them.

Creating an Alcubierre drive isn't just a matter of engineering. There's science that needs to be done to confirm that it's possible to create such a mechanism.

The fields required for an Alcubierre drive is (at least for the moment) just as speculative as "white holes" or Einstein-Rosen bridges, which can also be described in the context of General Relativity.

Given the ubiquity of the materials for life (Water, complex organic chemistry, etc.) in the universe, it seems incredibly likely that life does exist elsewhere.

Given the enormity of the universe, it also seems possible that multi-cellular life exists elsewhere too.

That such life could evolve with intelligence and a technological civilization is a much more iffy proposition (we only have n=1 WRT both life, multi-cellular life and intelligence).

There's also the problem of scale. Given the time scales involved (~14 billion years), if (apparent) FTL locomotion (whether via an Alcubierre drive or some other mechanism) is feasible, and there are other, more advanced technological civilizations in our galaxy and/or the larger universe, there should be hordes of aliens everywhere (cf. Von Neumann probes[1]).

Since we don't see that, it points up our ignorance.

All that said, yes it is possible (although unlikely in the extreme given our current understanding) that we have been visited by intelligent, extra-solar aliens.

But in the physics we do understand (almost certainly flawed and incomplete), anything is possible, including time travel into the past (with certain constraints).

But what's possible isn't what actually is.

I would be fascinated and very excited if/when we discover the existence of life (even single-celled organisms or some facsimile thereof) somewhere other than the Earth. I believe that there's is almost certainly life elsewhere in our universe.

But what I believe != what I (or anyone else) can prove.

Unless and until such proof exists, I will remain (random youtube videos notwithstanding) skeptical.

And so I draw your attention back to what I said in the comment to which you replied[2]:

I'm going to wait for that "Klaatu barada
nikto[1]" moment before I buy into any claims of
intelligent aliens visiting our little world.

Which is to say that until some actual, provable, evidence exists, I'm going to hold my breath.

[0] https://en.wikipedia.org/wiki/Independence_Day_(1996_film)

[1] https://en.wikipedia.org/wiki/Von_Neumann_probe

[2] https://news.ycombinator.com/item?id=30595469