From my extensive browsing of their website(/s… less than a minute… ) I couldn’t find their server/sync component being open sourced. I did see they open sourced all their clients though.
They have most of the server open sourced and have it in their roadmap to go full open source. That said what isn't open source today isn't open source and they don't have an actual track record to open source things they haven't before so YMMV.