[jiggy2011]

They probably didn't mean that they "know everything there is to know" unless they are very arrogant.

Perhaps they meant that they knew the syntax to at least one programming language and where at a point where they felt it would be easy to learn more if needed but they no longer felt the need to study it as an end in itself for their purposes.

having said that it's easy to overestimate your level of knowledge when you haven't been introduced to anything more difficult.

You may believe for example that you can easily solve say traveling salesman because you figured out how to write the naive n! solution and tested it with 5 nodes on a fast computer and it worked fine. If you never have to test it with 1000+ nodes then you may never find the performance inadequate so will never have to open the dynamic programming rabbit hole.