Torah Temimah cites Tosefta Shevuos 3:6.
“And he is a witness” — one fit for testimony; “and he heard” — to exclude the deaf; “or saw” — to exclude the blind; “or knew” — to exclude the deranged; “if he does not tell… he shall bear his iniquity” — to exclude the mute.
Each clause of the verse is read as a limiting term that removes a class of people from the shevu’as ha-eidus liability structure.