At my last employer before retiring (not tech but used a lot) has a very unique (and way larger than Twitter) complex set of businesses. They change at an insane pace and often involve things that in the end don't ship, resulting in crazy complex code base networks. We also had 100's of teams building every kind of software imaginable (server api's, web apps, mobile apps, internal apps, hardware with embedded code, etc). Anyone from the outside coming in cold to examine the code would have no idea where to even start, much less be able to evaluate anything. It's not that any individual thing was necessarily complex, but there were so many interconnected business practices and related businesses that understanding how they relate is very hard for anyone who has been there for years much less someone from an unrelated industry.

For example you could look at my team's mobile codebases and probably figure out what was going on, but understanding all the services we consumed, and what they consumed, etc. (given the deep mix of micro services and macroservices) would make understanding the why of the entire system impossible.