By definition, it could be more secure if people actually would be able to inspect it. Is it regardless the most secure? Hard to say. GrapheneOS is quite secure, probably more.

GrapeheneOS are both operating systems where a single party has the signing keys to run any software they want on your device at any time, and even remotely brick it.

I for one am not comfortable with anyone having that much power over something that is supposed to be mine, so they are the same to me. Untrustable black boxes.