I was on CX845 on 3/26 from JFK to HKG. ET was showing availability (i.e. flight wasn't zeroed out before departure) across all classes, however, my flight ended up completely full on the date of departure as ISM told me. Therefore I think what ET currently showed isn't at all accurate as many pax like OP (or his dad) is still trying to rebook. When I called CX about my original flight cancellation (which I was supposed to fly on 3/28), they told me I was protected on 3/26. The other 2 flights, the last two out of JFK, on 3/29 and 4/3 were completely full.

My flight ended up being delayed to the next morning given the restriction on T8 departure passed 8pm as someone mentioned above. In J, there was a full set of cabin crew occupying. As a result, CX upgraded me and my daughter to F.

To answer OP's original question, I still see ET availability of CX841 on 4/4 showing per below, but I highly doubt this is accurate given the fluid situation.
F4 A4 Z0 J7 C6 D6 I5 U W9 R9 E9 T0 Y9 B9 H9 K9 M9 L9 V0 S0