It is absolutely not alarmist. Seriously, go apply to be a Homebrew developer right now under an alias, make some useful updates for a few months, then submit a PR as another alias, and approve and merge your own changes from the first alias. After you get away with skipping code review a few times, now go switch a popular package that relies on abandoned sources to point to your fork that makes a few legitimate updates.
Now in a few months make an update that compromises the random number generator on every brew users laptop making it easy to covertly intercept and manipulate the traffic of any target however you want. Maybe for less chance of getting caught, target sysadmins or release engineers by targeting a niche tool many of them use and wait until they publish new ssh keys to github (which are always public!), that you can re-generate the private keys of since you know about their compromised entropy. Check for public dotfiles of your targets to pick the right niche tool. Now go collect database dumps or crypto from a pile of major companies. Easy money if you do not have ethics.
Seen these attacks in the wild many times, some at close range. A bored teenager could do this, and in fact attacks like this happen every day. And everyone just says pointing it out is "alarmist". People used to defend lead paint and asbestos too. Survivors bias is a hell of a drug.
I have done supply chain attacks along the lines of the above myself to prove the risk is very much real to my clients. It is expensive to fix so wanting the risk to be overstated is understandable, but it is actually super easy. Sometimes you can even get away with buying the expired email domain of a inactive maintainer so you can easily just take over their identity. Done this twice and I am no state actor.
Nix, by the way, is hash pinned, but you just blindly update those hashes when there are updates, right? Their code submission process is as yolo as Homebrew. Any rando can push whatever they want without multi-party code signing. I am the one that submitted the RFC to nix to fix this which was rejected as in the end they admitted they want a hobby distro and do not want the overhead and rigor needed for professional targeted environments.
This is why I was forced to abandon nix and get all my clients to abandon it, in favor of StageX which even as the founder I have no control of. It mandates deterministic builds and cryptographic signing by at least two well known keys of maintainers.
We do not point out risks unless there are solutions to mitigate them.