From the RNC resolution:
"A person's sex is defined as the physical condition of being male or female, which is determined at conception, identified at birth by a person's anatomy, recorded on their official birth certificate, and can be confirmed by DNA testing."
In other words, the Republican Party says sex is immutable and determined at the moment of conception.
So actually the national Republican Party position appears to go considerably beyond most of the bathroom bills introduced by Republican legislators at the state level. For instance, the Virginia bill appears to provide for an exception where a trans person has undergone GCS and now has genitals conforming to the person's gender identity. Under the national policy, even that wouldn't be enough, because you still have the DNA you were born with.
Next step? To make enforcement of the bathroom regimen easier, everyone will be required to have "M" or "F" tattooed on his/her forehead at birth?