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The Aircraft went into Alternate law as a result of the iced pitot tubes. In Alternate and Direct Law the stall warning is now based on AOA only as the computers recognize the IAS may be wrong ( amongst other things) .
Seems like a good fall back thing to me.
If the AoA system requires pitot input in order to compute the AoA and if the pitot(s) become iced otherwise non-operational the AoA figure will be wrong.