Darth Luke
Sith Master!!!!
I think Sifo-Dyas was indeed a real jedi, who was killed 10 years prior to Obi-Wan visiting Kamino.
What I'm trying to say is that the alien was struggling with the name of the person who ordered the clone army. He might have been struggling to say the name "Sideous" but in his mispronunciation Obi-wan assumed he said the name "Sifo-Dyas".
Imagine someone who speaks a foreign language trying to say a name and you here a few fragments and assume they mean "Tom" when in fact they mean to say "Tim". Then you say, "oh, do you mean Tom?" and they're like "yeah sure whatever, I don't understand the language very well".
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Taun We just happens to mispronounce "Sidious" as "Sifo-Dyas," which just happens to be the name of an actual Jedi who died 10 years ago? That's a bit of a stretch, if you ask me. And I believe he called him "Master Sifo-Dyas," and Sidious usually goes by Darth or Lord.
The right explanation is probably the most simple one. I think Sidious or Dooku posed as "Sifo-Dyas" when they ordered the Clone Army. If the Kaminoans had never seen that Jedi before, then how would they know he wasn't telling the truth? As long as whoever ordered the clones paid for them, I wouldn't think the Kaminoans would care.