There is a difference between sleep sex and rape. Since we have zero context as to whether whoever is fucking her is a lover or is actually doing so against her will, it cannot be determined which it is. They are not mutually exclusive, of course, but they are not the same and sleep sex CAN be consensual.
It's not ok to assume consent in game or in real life. If it isn't explicitly given in game, then the player can't know if consent was given or not, and has to play the game with the distinct possibility that they are committing a rape in the game. People who don't like that should know that without having to play the game, hence a rape tag. If there is explicit consent given in the game then I withdraw my request for the tag, but given that the games one sentence description uses the word "defenseless" to describe the girl - a word generally used to describe someone or something that you can take or aggress upon freely even against that person's will - I'm going to guess that consent isn't given. Happy to be proven wrong though.
Another indicator is that the girl is taking a nap, possibly in public, not sleeping at home with the protagonist at night, that lowers the likelihood that the MC is a lover given blanket consent to have sex whenever and wherever while the girl sleeps. Also you generally have to be able to withdraw consent for it to be valid, and given you can't do that while sleeping and that option is never given to the girl in the game it's almost certainly rape regardless.
Also your argument could be extended to say that ANY game with rape isn't necessarily rape. For instance, what if all the characters in the game are porn actors who all gave consent offscreen before the game started? You can't know it didn't happen so no games should have rape tags, because we can't be sure if it's rape.
TLDR: If you don't receive consent it's rape, game or not. Arguing that stuff could have happened offscreen is illogical and can be applied to any possibility ever, making any game or narrative tagging meaningless; you have to go with what is in the game.