The Javadoc states that:
"Note that the thread that writes packets will be used to invoke the listeners."
If you know the Smack internals you will know that there exists a dedicated writer thread.
If you just look at the PacketWriter class you will discover that the "sending/writer" thread
is the thread that just called Connection#sendPacket(...) and so it will be responsible for the
invocation of the listener callbacks.
It is just a little bit confusing 
Furthermoore the Javadoc reads:
"The listener will be notified of every packet that this connection sends."
This is not true. The listener will be notified about every packet that the connection is about to send, because
calling XMPPConnection#sendPacket(...) will just add the packet to the internal buffer. Afterwards it is still possible
the the connection will break and so this packet will never hits the wire at all.
BR