I'm a former Confluence developer (2008-2013). I don't know anything about you or the detailed history of Confluence.
- When I started there were 12 people on the team including me. I wasn't there at the very beginning but some people on that team were and they would discuss design decisions from the start of the project.
- It was a spring-hibernate-webwork Java web application. Very complex and structured. Not your typical weekend project. Mike Cannon-Brookes (co-founder) of Atlassian wrote a book on this type of architecture.[0]
- They were always very open and forthcoming about open source they were leveraging. I don't remember this ever being discussed or mentioned.
[0] https://www.wiley.com/en-ie/Java+Open+Source+Programming:+wi...