Whos to say he wasnt?? The Dark Side clouds everything!!
but on a proper not. It still comes down to Yoda, ObiWan, & Mace Windu seemed to know about this Sido-dias from 10 yeara ago. Which makes you wonder :
A) Is the Jedi Council THAT Blind that a false person has been made to appear as if they they where there all along?

Sido_Dias was a previous apprentice of Darth maul (Not likely) or Darth Sideous ( Even less likely due to the 2 Sith rule)
But, What about if Sideous played both sides of them (Maul & Sido-Dias)? Without the other the other existed? Maul could have been a front, where as Sido worked in the background?
Not likely Ill grant.
Personally, & I have no proof for this, This is just my thoughts) Darth Sidous placed the order for the clones (Using a semi false name) knowing that after the Trade Fed would / had gotten beaten, the something would be needed to make have these 'Emergency' powers; After all, Its pretty obvious oth Him & That Blue skinned guy with horns where playing Jar Jar int their hands.