You're missing the point.
TCAS is not a box that causes things.
It is a system - involving displays, ATCOs and pilots.
TCAS did not fail during the DHL incident - the system did.
And if you re-read Nigel's post, he states that the system caused the accident - and he is correct about that.
But it does tell you about where other traffic is in both azimuth and relative altitude, so the resolution is obvious
It's not really that clean cut. And you are only quoting on a head-on situation. With no RA/TA commands, on your "TCAS" display - what would you do in a situation that was not head-on?