Yes Epic will replaces Xsolla for Backer Builds. Which is a good thing, because the Xsolle servers were overloaded when a large number of backers download the game. You must be aware that there are more than 200k backers for BB.
How does this affect early access through the backer builds?
Our webstore at http://store.phoenixpoint.info will stay open, and you can still get early access there with the Platinum and Signature Editions. Future Backer Builds from Backer Build 4 onward will be delivered via the Epic Games Launcher when you redeem your key.
https://phoenixpoint.info/epicfaq?_ga=2.239176970.864881939.1552474217-1622525914.1529534897