In Christian tradition, no human can ever be faultless by nature, so it seems a bit daft.
Not until Augustine won that argument with Pelagius a bit later?
Anyway, it's difficult to know what the Donatists exactly believed because almost all of our evidence comes from their orthodox opponents, but I don't think the Donatists argued that priests were required to be
faultless in order to administer the sacraments. My reading is their doctrine was that priests who had lapsed under persecution and renounced the faith
had to be reordained, and if they weren't then their sacraments were invalid.
At the Council of Carthage in 411 the Donatists agreed with the orthodox party that the church contained sinners, but that if the sin was known to the church then it had to be "threshed out" through some kind of penance for a priest to continue to legitimately hold office. The source of the controversy wasn't that certain churchmen had sinned during the persecution, but that even though their sin was publicly known they had never admitted fault or atoned and were still ordaining each other.