"Many null modem cables do little more than trick your computer into thinking all the appropriate signals are present (and then swap transmit and receive data)."

It is not correct to call such a cable a "Null Modem". A true Null Modem cable must pass-through (and cross where required) all the interchange circuits - both data and control.

Unfortunately, as has already been noted, this doesn't stop people mis-using the term "Null Modem" - and possibly even selling you something which isn't really a Null Modem! :-(