Yes. Emby was originally open source, but people would regularly fork it to remove the licensing. When they chose to go closed source; jellyfin forked that final release and has built from there.
Emby has a premier licencing system to support their development, instead of selling user data and making deals with content providers like Plex, or depending on OSS development/contributions like Jellyfin.
As far as I understand almost 80% of jellyfins current code is the original Emby code (called 'media browser' or 'MB' at the time), though to be fair, I haven't verified that claim.