And so, if a person consciously admits they have internalized racism, or have subconsciously internalized racism in the past, we therefore know that other people have subconscious, internalized racism?

Somebody who claims to have been subconsciously racist is the only relevant argument. Conscious internalized racism means nothing towards subconscious internalized racism. The conscious racism is a behavior that can be seen, and subconscious racism is just a claim thrown to attack opponents. You cannot actually prove the existence of subconscious internalized racism by testimony from someone who was consciously doing it.

People can not know they were subconsciously racist until after the fact, meaning that they must not presently be subconsciously racist if they are admitting to it. This 'past state of mind' testimony is worthless, because you might as well say they are presently subconsciously internalizing anti-racism bias. It's the same logic being used to demonize black people who disagree, and it's just as falsifiable.

So yeah, some people can say they are internalizing racism consciously. That doesn't mean somebody is subconsciously internalizing it.