P. Bo - V'yameish Hoshech Onkelos, Rashi and Sinai

See Bo 10:21 "V'yameish Hoshech"

Rashi:
k'mo v'ya'ameish...

Then he says
V'Onkelos Tirgeim l'shon hassarah k'mo
"Lo yamish"

If Targum Onkelos is miSinai how can Rashi argue and say
"Ein hadibbur m'ushav al havav"

If Rashi MAY argue - then what does it mean to say Onkelos is MiSinai?

Shalom
RRW