No, it's what's called people not understanding the plot.
The Sith convinced the Mandalorians to start the Mandalorian war. At the end of the war, when Revan and Malak had gone bad, they went hunting for the Star Forge. Once they found it, Revan would have slain the Dark Lord of the Sith and taken the position himself. It makes perfect sense.
I'd hardly see the Sith who convinced the Mandalorians as some strange, mysterious faction. It was probably just whoever was the leader of the Sith before Revan took over.
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Wrong.
If it contradicts EU, then it isn't cannon. Obviously, you haven't read Tales of the Jedi. Otherwise, you would realize that your half-assed explanation actually contradicts the events following the war with Exar Kun. There simply was no 'empire for the Sith to retreat into' as Canderous explained.
'Tis false, counterfactual, erroneous, inaccurate, specious, unsound and untrue. :D
After all you don't earn the title of "Bio weenie" for nothing.