I believe it is similar ot other crafts. You won't be a better engineer by only looking at source code. But it does help when you are analyzing a codebase to learn something applicable to your situation. So, it is not "just reading" I believe, but a subconcious act of analyzing.

Like with any advise that ought to get you expertise or ability, if you're not goal oriented, actively gathering and acting on feedback and hyper focused on improving, you will only waste time.