The manual page says:

       sendfile() will transfer  at  most  0x7ffff000  (2,147,479,552)  bytes,
       returning  the  number of bytes actually transferred.  (This is true on
       both 32-bit and 64-bit systems.)

I understand that you can pass a larger length, but a single sendfile() function call will never copy more than 0x7ffff000 bytes. I see nothing wrong here.

If you want to be sure, try to copy more bytes than 0x7ffff000 and see what happens :-) Use strace to check system calls.