I expect Wei's intuition is that knowing self means having an axiomatic definition of (something sufficiently similar to) self, so that it can be reasoned about for decision-theoretic purposes. But if we look at an axiomatic definition as merely some structure that is in known relation to the structure it defines, then your brain state in the past is just as good, and the latter can be observed in many ways, including through memory, accounts of own behavior, etc., and theoretically to any level of detail.

(Knowing self "at a low, instinctive level" doesn't in itself meet the requirement of having access to a detailed description, but is sufficient to point to one.)