I'm afraid all those suppliers are simply wrong then and it hardly seems fair to blame Mongoose for their mistake.
I have not seen it said ANYWHERE that you don't need SFoS if you have the revised rulebook.
And as for the supplement not containing the rules needed to play it? Armageddon does contain any new rules needed to play it, it simply doesnt reprint all the existing core rules, the updates in SFoS are more or less ACTA 1.5 rather than a supplement, the fact that the suppliers you asked got this wrong is of course rather poor on their part but it is worth noting that strictly speaking you don't NEED SFoS OR Armageddon to play against opponents with either set of books, you simply wont be able to pull some of the more advanced tricks (like boarding or skindancing).
As LBH notes though I can't recall seeing ANYWHERE saying that revised edition contains all of SFoS's addittions, it merely says that it is fully SFoS compatible (ie it includes all the revisions just not the extra ADDTIONS). (And yes I know this could be clearer but it's still not incorrect)