Sayyiduna Ali (may Allah be pleased with him) says: When a person has sat in the prayer for the length of the tashahhud and then (after that) loses his ablution, his prayer is complete.
The Commander of the Faithful, Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), would never go against the hadith of the Prophet (peace and blessings be upon him). Even if this narration were authentic, the proof would still be the narration that he or someone else transmitted from the Prophet (peace and blessings be upon him), against which the saying of no member of the Ummah can serve as proof. The second answer is that this narration itself is weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2963