The question is not whether a determined adversary could corrupt the counter. The question is whether they can corrupt the counter before they corrupt something else that causes total data loss.

Physical defenses are not security through obscurity, and why you assuming they don't use known ciphers?

Kerckhoff's principle should be adhered to if truly secure encryption is desired; alas, then all sorts of hard obstacles pop up (UX becomes a SPOF, most commonly - a secret always needs to be stored somewhere, if only in the user's head).

OTOH, the practical purpose of encryption is to remain unbroken for long enough, not to be completely unbreakable. As seen here, security-through-obscurity is practical enough in cases where user-obtained key material is too weak to provide enough protection using strong publicized crypto. In other words, it's a two-part key: one is in user's wetware, the other in phone's hardware (as per obXKCD, it's usually easier to attack the former).