That was only half serious - I know that are valid use cases for people to prefer using binary distros. However I think this particular issue is a good example why IMO even binary distros need to provide a convenient option to locally build any package for security conscious users.
In other words, if you don't know your compiled binary is the same as the distributed binary, you have no reason to think yours does not have a vulnerability added by the toolchain.
Unless I'm the one that is misunderstanding, of course. :)
In fact, if you compile it yourself, unless you can verify the compile against a "known good" one, then you can't even be sure that your local toolchain hasn't been compromised. (I mean, sure, if you were a perfect auditor of your entire toolchain, then you could have some confidence here. You have to be perfect, though.)
Consider, you do a compile of Firefox and it is different than the one for download. Why? As things stand now, you don't know. And that is the problem.
You do more to protect yourself than taking the same vulnerable source and compiling it with Mozilla's "reproducible build chain".
If the source itself is corrupt then having a verified build of malicious source is completely useless.
With Gentoo you can verify the source itself matches the "trusted" upstream source and then build it with your own trustworthy build chain.
And before you go "what if your build chain isn't trustworthy huh????" think about it a little further... if your own local build chain can't be trusted you're already screwed even before you download anything from mozilla.org, just as you'd be if you downloaded a "bit verified" binary from mozilla to run on your already-pwned local operating system.
You do protect yourself from vulnerabilities in their toolchain. And this is where the effort makes sense. If there are differences in the builds, then you can at least suspect one of you has a tampered environment. Right now, you have no way of knowing that one way or the other. You just have the joy of having done your own build.
My main question is still just one of magnitude. Consider, I have not had a wreck or other car mishap in 20 years. I could conclude that seatbelts, then, have not increased my safety really. I am not trying to make that claim, as I feel it is false. So, my question here is essentially, how much safer would this really make things? (Or trustworthy, if you'd rather that term.)
The only sane way to help these people trust their software is to enable meaningful third-party audits of said software. And that requires that the auditor be auditing exactly the same thing as the user is using.