If the incident took place when the aircaft were receiving an ATS from Terminal Control and the minimum horizontal separation was 3 miles, then there is no airprox.
TC are authorised to provide 3 miles or 1000'. I take it Charles' plane was on a LACC frequency?
Also 900' separation is within the tolerances of Mode C variation. One aircraft can be maintaining FL120 and displaying FL118 and another maintaining FL110, displaying FL112 - that gives only 600' separation (on radar) and no airprox.
And another thing - The Sun said it was so serious that both pilots filed a report! is that both pilots on the 146 or the A321? Media always like to assume one pilot per aircraft don't they?
I expect that it was actually pilots of both aircraft and 2 ATCOs who ended up reporting. That must make it deadly serious!