it wasn't in the original web novel but it is a important volume, the manga adapts the Light novel not the web novel so it should have been adapted, Sara, Soldat and the party rudeus was does appear again in the future and the adventure he had with them is a important part of rudeus development, he didn't just skipped it he resumed it in 3 pages as a joke, completely different from what it really is, seeing people treat rudeus depression and erectile dysfunction as a joke proves that the lack of this content affects the experience, why do you think it was added to the light novel if it hadn't a purpose?
the author himself said that he regreted to do a time skip in the original web novel and fixed it in the light novel.
and volume 7 is considered one of the best volumes of mushoku.
there is literally no reason to skip volume 7, its just a bad decision

the thing is, it was portrayed in the pov from the rapist, it was made as a "fun" thing, because they were having fun raping her, and as you see it, you think "how the fuck is that fun?" and just makes the scene gross, that was intentional, this is the same thing as putting classic music in a warfare.