For server side, one cares whether each connection can read or write, not the listening server socket. The listening socket does not read or write.

This is false, and your example is not analogous to the original post. A listening socket becomes readable when a connection is ready to be accept()ed. His code correctly tests the listening socket and all open client connections, reacting to a readable state on the listening socket with an accept(), and on all other sockets with a read. (The read isn't quite right since it blocks waiting for a full line, but the intent is fairly clear.)

Your version blocks on an extraneous accept() before hitting the select(), and is unable to ever accept any more connections. Your "simple testing" fails because it doesn't select on the listening socket until after it pulls the first connection off of it. So, yes, that select() doesn't fall through until a second connection comes in. His code doesn't have that flaw.