That wasn’t at all the case. Aside from Archimonde (who’s implied to have likely already been communing with Sargeras through Thal’kiel’s skull beforehand), the eredar thought the deal they were making with Sargeras was the opposite of what they actually received.
The Dark Titan posed as a shining golden figure offering to grant them the means to benevolently share their knowledge and civilization with the rest of the universe, by which they could uplift other worlds’ peoples to advance like their own, then instead he transformed them all into corrupted monsters who would revel in the torment and devastation they wrought upon countless worlds.
Joining the Burning Legion was never presented to them as part of the deal because the eredar didn’t even know Sargeras’ true nature or that the Burning Legion existed until after they were turned into demons, which came with spontaneous mental conditioning that turned them into the sort of cruel monsters who would be eager participants in its cause.