Even if we treat her as omniscient as infallible, despite having examples that disprove that view, she didn't know that was possible (or at least, we have no reason to believe she did, and if she did you'd think she'd have been as careful to make sure all Forsaken die by balefire as Rand now is. ) There's still a window of opportunity unless the balefire is fairly powerful, however impractical and unlikely it is the DO will have the prior knowledge to take advantage of it. And, rightly or wrongly, I take "Sammael is toast" at face value; it would be very cheap, to me, if an author told his readers point blank that a character was dead just so he could shock them with a resurrection (and before anyone decides to scream hypocrisy, the bible makes clear as far back as the OT that Jesus would rise from the dead; it was only a shock because people didn't do their homework nor listen to Him when He said that's what would happen. )

Also, there are a lot of things we don't know about Mashadar, such as what happens to the people it consumes. Do they get spun about by the Wheel again, or do they become part of Mashadar? Could the power of Mashadar, which is the equal of the DOs at least within Shadar Logoth (hence the Cleansing) prevent Sammaels reincarnation whether or not it made him part of it?

In short, while I can't say definitively that we'll never see him again, I think the same is at least as true for statements about Be'lal, and still think he's the one who's coming back for one more round.