The concept of a polyglot[0] has been around for
ages: the Wikipedia page mentions an 8-language one going around on Usenet in the 1990s. If it works for source code, and for scripting languages, and for things like PDF+ZIP, why
wouldn't there be a pretty good chance the various executable formats are flexible enough to allow for it too?
A libc which is flexible enough to select specific code paths depending on runtime conditions? Every libc already does this to make use of the latest hardware features[1], using the same approach for platform-specific code isn't a huge stretch - you're basically doing a lightweight WINE.
Her work is definitely impressive, but it isn't magic. You'll see similar stuff if you look into the demoscene, or the IOCCC, or a decent chunk of the talks at CCC. And it's not like APE and Cosmo libc are seeing massive adoption: the people who want portability but can't even compile for multiple platforms are probably happier with something like Java due to the better ecosystem support.
[0]: https://en.wikipedia.org/wiki/Polyglot_(computing)
[1]: https://www.phoronix.com/news/Glibc-More-AVX-512-October-202...