In order to get to that answer we need to first abolish the idea that all rape is about power and violence. It’s not. Some rape begins as the earnest belief that sex is going to happen, and that it should. The confusion starts with misreading socially accepted cues. Like, for instance, the cue that says, She’s dressed in a way that I find sexy, and she’s flirting with me, so that means we’re going to have sex. That is not an illogical conclusion. A lot of times, that’s exactly the case. But not always.I can easily explain why this rape (assuming her friend was telling the truth about everything he did) was about power and violence. He physically violated her body, that's violence. And he specifically waited until she was powerless and could not say "no" or resist, in order to violate her. That's power. When people say that rape is about power not sex, what they mean is that when you rape somebody it is because you've decided what YOU want (even if that's sex) is more important than what THEY want (consent, body autonomy, etc). You are attempting to assert power over them.
Feminists say that rape is about power not sex, because they see the issue as about people's entitlement to other people's bodies, overriding their consent, and the desire for sex is just the detail. The reasoning is that if you focus on the sex part, you end up with people blaming women for teasing men, or men for getting hard and sending the wrong message, or people saying that if men just had more sex they wouldn't rape, and apologists for prison rape who say the problem is that hetero prisoners just can't get sex.
When people think that rape is about sex, they try to address it as if the problem is somebody's just not getting enough sex or is being "led on". For them, since rape is about sex, they start with the premise that this is all the fault of sexual drive, and try to address it either by satiating the sexual drive, blaming others for provoking the sexual drive, or trying to eliminate either the sexual drive or the behaviour, dress or persons they believe caused it. This type of thinking leads to victim blaming, shaming of sexuality (like homosexuality, which people attribute to male/male rape and pedophilia), laws that try to cover women up or control what we can wear, and all sorts of other things that miss the real point: we need to tell people that what they want does not give them any right, any justification, any excuse to force that on another person's body or will. And ultimately, focusing on sex ignores all the other reasons that people rape: revenge, anger, sadism, to humiliate, to torture, etc etc...
It's like if people trying to stop murders say "we should focus on making sure people don't cheat so their partners don't murder them", not only does it ignore all the other reasons/excuses why people murder, it also ignores that murder is about somebody thinking they have the right to take your life because of how they feel/what they want.
Rape is about power and control because it is about exerting what you want to do to a person's body and completely ignoring whether or not they want this. It is about controlling their body without their permission for your own purposes, be that anger, sadism, revenge, sexual desire, whatever... it's violating another person's body for what YOU want.