is there any truth?

do michele and sebastiaans writings give any legit teachings, even if theyre all stolen?

I personally find quite some value for beginners in the Vampyre Sanguinomicon of Father Sebastiaan, especially in the section of basic energy manipulation techniques. Michelle's Psychic Vampire Codex is in my opinion a bit more difficult to read, but basically contains similiar information.

So, to me the bottom line is: Are those writings at the same level as the Asetian Bible or similar literature? - No, certainly not.
Are Sebastiaan's and Michelle's books "worthless" because of this? - No. Especially Sebastiaan has copied from lots of other sources (not only the Asetian Bible, but also for example the Temple of Set), but those teachings are legit. Just don't go and buy fake fangs now because he recommends it )