Red Sexlinks are fairly frequently mistaken for males by other breeds. If she has a significant and red wattle and crest like most RS's do, she probably looks like a rooster to him, as it's more the crest/wattle that they react to than the feathering, generally. I know when my juvenile males reach a certain size of crest/wattle that the older males start noticing them.
In my experience high-production laying breeds and hybrids, whether Australorp, Leghorn, Isabrown or any one of many others often have very high testosterone levels as well, and often mate with other hens like roosters, crow, grow proper spurs, etc ---- but they're definite females, layers and fertile, some good mothers too. Even if she's not doing this she may be giving him the impression she's male.
If she keeps not challenging him he should settle down. If, however, he's separating her from the flock, her instincts will be to rejoin it, constantly provoking more conflict, in which case you may want to consider isolating him for a few days. Bullying's not ok.
Best wishes.
