I'm trying to figure out where you get the idea that the contents are passed to tail's stdin. It's not. The full path to the file (as given by the shell - full path if searched via PATH, relative path if PATH not used) is passed in.
Compare the output of "strace ./a" and "strace /usr/bin/tail -n2 < a". The former has a call to "open" and all reading is done from filehandle 3. The latter doesn't call open, and all reading is done from filehandle 0.