[Jan-Poo]

There's no need for an interview to confirm what is esplicitly shown in Saikoroshi-hen.

Spoiler for Saikoroshi-hen:

In that extra chapter of Higurashi it is indeed said that Frederica Bernkastel and Rika separated. You can see it as a somehow an even better ending for Rika, so that she can live a normal life after the events of Matsuribayashi.
Though Frederica Bernkastel "existed" even before the split so I don't know what that quote is talking about.

That fact described in Saikoroshi-hen lead to the theory that the split Frederica Bernkastel is the Bernkastel we see in Umineko. However there is no clear confirmation.

Ryuukishi confirmed that Bernkastel does work as a connection between Higurashi and Umineko, but he didn't specify how.

here is where he mentioned that

This is the reason he created Bernkastel in order to put a direct link between Umineko and Higurashi.

The interpretations vary depending on your take of the metaworld and the "sea of kakera".

Spoiler for Umineko ending:

If you think that's real, then you can conclude that the many kakera from "Umineko" and "Higurashi" exist in the same "sea" and that Bernkastel (originating from Higurashi) can travel through them at will.


The other interpretation is that the metaworld is a mere metaphor of the collective imagination, and that the various kakera of the "sea of kakera" are just fictional stories and not actual alternate universes.
From this perspective Higurashi in Umineko would be just a story in which the character Bernkastel was first created. And Ikuko would be the author that created her and named her after her cat.

There might be other theories but these are the ones I know. Anyway what matters is that neither was officially confirmed or disconfirmed.