The joke here is a brilliant double entendre about inheritance. The first person mentions inheriting a COBOL codebase last touched by someone's mom in the 90s (literal inheritance), while the reply points out that's not how programming inheritance works (you know, the OOP concept where Child extends Parent, not where your actual parent leaves you legacy code).
Nothing says "congratulations on your new job" quite like being handed 30-year-old COBOL that nobody understands anymore. The real inheritance tax is the mental breakdown you'll have trying to figure out why everything is in ALL CAPS and what PERFORM VARYING actually does.