First, Capn, you don't know how close they ended up. As the report says, the TCAS derived data is merely "indicative" - a word that means ... what, precisely?
Secondly, you don't know whether the aircraft would have ended up 'less close' if the jet had merely continued its approach and XGA had been left to give way once XGA was alerted to the jet's position on base. The actual response may have made things 'worse'.
You say that "things should not get anywhere near this close". What is the minimum distance you apply when engaging in amateur ATC?