I don't think the parent's point about handling 10k lines of code has to do with with ability to load these files into memory but rather about managing the complexity of such projects. When a program becomes this big, it becomes harder to keep track of all the names of variables, the argument types of subroutines etc. IDEs like Netbeans or Eclipse have autocompletion functionality that make your life as a developer at lot easier.
It's possible of course that Emacs or vi provide similar functionality but the main point is that you need some type of IDE when managing a large, complex development project.
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