That is incorrect. The OS and hardware can be designed to leak everything. Although perhaps the encryption could be done on a USB dongle mini-computer (plugged into the user's computing device) that is running custom open source OS and hardware.

What is very hard is to hide that the OS and hardware has that capability. Easier to hide if closed-sourced. But knowing it has that capability won't help us prevent it from leaking the data. Well I guess it could if we refuse to connect our device to any network. But who the hell is going to use a mobile device that is never connected to any network.

The only technical solution is to open source all the OS and the hardware. But we can't open source the hardware because the foundries can be subjected to a national security gag order.

The only non-technical solution is political. And the politics won't be improving until the boomers are gone. If Apple loses the case and the government mandates a backdoor on encryption, then computing is fucked. Because any backdoor can be hacked. The tech giants are all fighting the government on this. It would lead to a computing Dark Age. If they explain it properly hopefully the political support exists to stop the government.

But that might not even stop the NSA from handing out national security gag orders and finding some way to put a backdoor in the hardware.