Copy-pasting code without understanding is functionally same as importing large libraries or linking to them. You'll have same problems even if you didn't copy anything! This is why good programmers review code and prefer open source.
His advice about copying is entirely wrong, because even if you don't understand some code, it can still be okay to use it in your project.
The biggest problem with copying is that you easily get more lines of code you have to maintain. This is especially bad if you copy large chunk of code, which may improve later. If that happen you have to do the copying again to get the improvements. Link or import the code you need instead!
His advice about copying is entirely wrong, because even if you don't understand some code, it can still be okay to use it in your project.
The biggest problem with copying is that you easily get more lines of code you have to maintain. This is especially bad if you copy large chunk of code, which may improve later. If that happen you have to do the copying again to get the improvements. Link or import the code you need instead!