It doesn't entail any of that. Traditional distributions like Debian allow you to install any number of library combinations, they just do so badly by requiring you to do so on different installations updated at different times.
Supporting the presence of multiple library combinations doesn't mean that you "support" all version combinations in the sense of supporting all combinations of their versions being used in the wild.
It just means when users upgrade they can do so gradually for some sets of packages at a time. You can emulate this on traditional distributions by having a single VM image you use for your browser, then cloning the image, updating packages, and using that one for your E-Mail etc.
Having a mechanism like this should mean less support is needed from the distribution, because system-wide breakages are less likely to occur.
You'll get the same bugs, but users can easily back out say an OpenSSL version with a security fix for the 5% of packages it breaks with, while retaining the fix for their browser & other high-risk packages.
For example, I currently can't upgrade my firefox version on Debian testing because it ">=" depends on a version of a couple of libraries that some 100-200 other packages on my system don't like.
In Debian testing this sort of thing happens occasionally and will get fixed sooner than later, but that it happens at all is just an emergent property of how the versions are managed. If I were to run the latest firefox with those new versions and not touch the other packages until they're recompiled both myself and the package maintainers would be exposed to fewer bugs, not less.
I'd get a bugfixed firefox today, and they wouldn't need to deal with bug reports about a broken upgrade, or firefox bugs in an older version users like me can't upgrade past simply because firefox happens to share the use of a popular OS library.