I would say it's pretty "special" and well written code if outsiders can come in and quickly understand it. You should have commended them for their work.
I was coming here to say this. I have definitely found myself reading a codebase that is easy to read and easy to follow, and mistakenly concluding therefore that its developers were not solving complicated problems.
That is true. They were definitely a skilled team that had written the system. However I still think most large systems can be reverse “design” engineered fairly easily by someone who is experienced.