A33Zab
Actually the logic isn't changed, the AoA sensor information is fed trough the IRU part of ADIRU i.s.o. the Air Data part, resulting in the AoA data remaining available to FWC in case of Airspeed < 60 kts.
When BUSS was installed it needed switching off all three ADRs and with that Stall warning would have disappeared, that is why they brought in a mod that gets AOA data through IRs, that's all. Whatever was happening through ADR now happens through IR. I do not think it has any connection to invalidity below 60kts. Yes you need BUSS for this MOD. The FPA has been modified through
MOI: 153528/P12909 (A320FAM)
Where with all ADRs loss baro-inertial vertical parameters are replaced by hybrid GPIRS parameters. As a result Bird is available and should be used in Unreliable speed situation.