As far as I can tell this only affects AUR(which you have to go out of your way to access and you are encouraged to review all the build scripts, although in practice I suspect many people are complacent or not knowledgeable enough), not the official repos. I am guessing the official repos for gentoo and arch have similar security, but if you go and get unnoficial/user ebuilds for gentoo you likely run into many of the same situations (although I don’t really know much about the gentoo ecosystem).

It is also worth pointing out Arch probably has a much larger target on its back since it is much more popular and there are many distro’s based off of it(which tends to attract the less knowledgeable users, who are primed targets for just randomly getting AUR without thinking)

Edit: For gentoo, it looks like GURU is more secure and has trusted users who review, but in practice I don’t know how well they do review or if they have had problems of people trying to submit malicious scripts. Also, it looks like GURU is considered more “official” than AUR so they are not really direct comparisons but I suspect they play similar roles in the community.