bitcoin is not a threat because the price is volatile and need stable like Tether while people not want to trade.but not for libra,its stable like fiat and can use for payment online/offline.
I think because Facebook has developed stable digital money if tether and the like are not a threat but Libra can be a threat to $ Fiat because Facebook collaborates with many projects so that its presence could be a threat to the dollar in the future.