Regardless of what distro you’re using, Steam comes with an Ubuntu 12.04 based runtime. steam-native is basically a launcher that runs
STEAM_RUNTIME=0 steam, disabling the Ubuntu 12.04 runtime and telling Steam to use the system libraries instead.
The arch package steam-native-runtime comes with all the libraries you’ll need to run steam with STEAM_RUNTIME=0, but some games require additional libraries.
I’m not sure what exactly the issue with Portal 2 is, but it seems that it ignores your STEAM_RUNTIME variable and tries to use the Ubuntu 12.04 runtime anyway. Which is why you need to manually load the additional libraries with LD_PRELOAD before launching the game.