The other things being mentioned are just some of the required 'defences in depth', and it is right to ask why they failed. The fact they did fail emphasises the need for depth.
What they comprise is up to MoD; DV merely points out that AM Garwood himself mentioned TCAS. Having been so firm and specific in a 6-year old annual report (which simply regurgitated what was said in the 80s and probably before), the key question is why the chosen solution has taken so long and failed, when another system was successfully trialled and recommended 30 years ago.
It seems that systems integration is the now problem. That's actually a huge leap forward, given previous losses due to flat refusal to integrate safety critical systems. For example, Tornado ZG710 (2003). The trouble is, this practice was by then policy, approved by Garwood's predecessors. His 2014 statement was very brave (in a Yes Minister sense).