If it can't get an encrypted timestamp signed by a particular private key then it knows it doesn't know what time it is

Probably it's like the Nintendo 3DS, the user facing clock is just an offset to the official internal timer, so when the user changes time, it's just an aesthetic change and has no effect to time/date game unlock mechanics (mostly lPokemon games). When CMOS dies, internal clock resets to 1970, a clearly invalid date where all the signing certificates are invalid, and the user can't set internal clock without hacking the console

My guess would be that it shipped from the factory with a "correct enough" time for the system to not care.
Had it shipped with a dead CMOS battery, the date would have been reset to 1970 or something, then it would complain.