Inlining it means that you're still executing the exact same code. If it had problems as a function, it still has problems when inlined.
But if it's inlined, it's no longer a function. ;)
What he's saying here is that the function itself was fine and free of bugs but that there is a problem for the programmer. The programmer's understanding and expectation of what the function does isn't in that it affects state in a way the programmer didn't know or expect. What the function does becomes much more obvious and controllable when you inline the function's body.
But if it's inlined, it's no longer a function. ;)
What he's saying here is that the function itself was fine and free of bugs but that there is a problem for the programmer. The programmer's understanding and expectation of what the function does isn't in that it affects state in a way the programmer didn't know or expect. What the function does becomes much more obvious and controllable when you inline the function's body.