One might say, yes. They haven't authoritatively expressed in that capacity, however, in a scene of The Flash we see Barry going through the Speed Force. While inside, he witnesses different universes, including a fix of the 90s Flash TV show, and Supergirl (who at this stage wasn't even a CW appear).
In the event that the Speed Force demonstrated these as different universes in the Arrowverse, at that point it makes sense that Smallville, the DCEU, even the Dark Knight and Burton/Schumacher Batman motion pictures all exist in their own universes/parallel universes and the Arrowverse (and Supergirl's Earth) is only one of many.
Presently, this raises the inquiry concerning why there be a doppelganger in a few universes, and in others, they appear to be unique. The unrefined (and truly feeble) answer is perhaps an alternate sperm got however same individual, yet unique hereditary show up.