I don't think anyone at Apple considered this LED to be designed "tamper proof". They just lazily implemented a requirement possibly written as:

-- A LED shall light up when the camera is activated.

But what they should have implemented is:

-- It shall not be possible to activate the camera without the LED being lit, taking malicious third-party software into account.

The first requirement is obviously, and demonstrably, implemented by every camera (ignoring hardware defects regarding the LED). The second is, as proven by the paper, not fulfilled by any.

{EDIT: added the malicious part to the 2nd requirement}