A discussion with the writer and director of the movie touch on exactly this:
When the people come back from vacation, and then the credits sequence with J.K. Simmons, that seems to me to indicate that they did. Is there ambiguity for you guys?
AS: If you asked me and Max and Samberg and Milioti, we might all have different feelings about the end. The most important thing to me is that they go into that cave together. That’s the emotional ending.
[...]
When the goat goes through and gets blown up, Sarah says it disappears in the loop. But Nyles doesn’t; he’s still there [when Simmons’s character returns to the wedding], just his mind is wiped. So was she lying about the goat disappearing?
MB: I think that’s kind of what we’re referring to about the end, and if they’re actually out or not.
AS: I’d say everything is there for a reason, from our costume design and wardrobe choice, and production design and props, the choice of drinks and things on the tables in the background. And then there are specific lines of dialogue … If you want to crack it, maybe the clues are there. But maybe not though, too.
So basically, the idea is that the science behind it is all still theoretical, and it's ambiguous to understand if it did actually work or not. It is potentially just an "Inception" ending - leaving the ending ambiguous and keep the audience guessing whether or not it did actually work.
