The fact is this,
a mistake was made, albeit a very serious one, however it was atleast done in clear visibility. If it were thick fog then the mistake may have taken a turn for the worse.
TCAS - only useful if the aircraft on the runway is squawking mode charlie. Certainly for general aviation, we don't flick the box to mode C until we are lined up ready for takeoff. for example - we are waiting fine on the taxiway as theres one inbound, flicking it to mode c would cause a warning and the pilots are trained to follow what the box says, which in this case would be a go around - thus wasting time.
Airliners use mode s ADS/B and so there should be a lot more info for the TCAS. I would say on this occasion, the pilots received a warning from the TCAS whilst simultaneously noticing the other aircraft and elected a simple go around. Even if a donkey stopped, it would climb away quite easily.
Ive had similar things happen to me, there's one guy at wellesbourne airfield who isn't particularly good in the tower and cleared someone for take off when i was 1/4 mile from the threshold - plus there were another 6 in the circuit - and one joining from the dead side - not a lot of room to move.
