They can't, because (responsible) security disclosures are private, _not public_. That's the whole point of the system: notify the developers in private ahead of time (usually 30, 60 or 90 days) so they can write, test and roll-out the fixes before you release the info to the whole world. This is to minimize the time between when bad actors gain access to the exploits vs. when users install the patch. So "keeping up on security disclosures" cannot ever be a 'pull' process.
Usually the maintainers of the big distros are part of (private) security mailinglists and receive such info. Just not in this case it seems.
Sending emails to some big distros would still result with e.g. Gentoo not getting that info because they are not a big distro.
Not ideal, but also: shit happens? It's always a balancing act choosing the lesser of multiple evils and most of the time it seems to work ok-ish, which is probably the best we can hope for ;-P
For this specific "bug" they took care to not mention any security angle in the commit message, making it extremely hard for an outsider to even realize this was a critical patch. I assume this was because they wanted to push the fix without breaking embargo.
The post you are responding to says that it would be nice if they copied literally one mailing list.
Expecting a FOSS project to go track down all of its (millions of?) users seems like a very unreasonable expectation, and is well outside of their scope of responsibility.
People have gotten so used to the Github flavour of free-labour, social-network-style FOSS that they've forgotten what all those LICENSE files actually say, which is to make it explicitly clear that the devs are not responsible to you for your issues, up to and including the software setting your house on fire. If you don't like it, you don't have to use it.