That's not the point.
The point is that, during anal sex, a bottom has a higher risk of catching HIV from a top than a top has of catching HIV from a bottom. The infection pathway goes easier one way than the other.
As you can see here the risk for the receptive partner in unprotected anal sex is 102-186 infections per 10,000 exposures, while the risk for the insertive partner in unprotected anal sex is 4-28 infections per 10,000 exposures. In simple terms:
The man being fucked has a 1.02% - 1.86% chance of catching HIV from a HIV-positive man fucking him.
The man doing the fucking has a 0.04% - 0.28% chance of catching HIV from the HIV-positive man he's fucking.
On average, the risk is 12 times higher for the man being fucked than the man fucking.