The issue, I feel, is with the definition of "racism" which he's using, which comes straight from the
Diversity Council Australia.
"Racism is when an individual or organisation with race-based societal power discriminates, excludes or disadvantages a racially-based person because of their race, colour, descent, nationality, ethnicity, religion or immigrant status."
By that definition, a individual without "race-based societal power" can never be racist. As Sam Kerr is of Indian descent, she can never, by definition, perform a racist act in Australia. In India, perhaps, but not in Australia.
Personally, I'm not a fan of that definition of the word, but that's the one he's cited in his apology.