Errr ... doesn't seem so ... the old version of this AdobeXMP.dll which went with Adobe Acrobat 8.3.1 still triggers the same "Nymaim.Trojan" detection for me. This specific dll version was "Adobe XMP Core 4.0-c321" i.e. version 4.0.0.321 (389,120 bytes). I checked more recent versions of this same-named dll installed with more recent versions of other pieces of Adobe software using it, and they have no issue.
Since my earlier post, my MWB had automatically updated to v 4.2.3.96 / update package version 1.0.32546 and component package version 1.0.1104, and I did restart as indicated before rechecking the AdobeXML.dll, which gets quarantined right away.
I am happy to set it as exception in MWB, as long as you kindly confirm that it is indeed a false positive.
Please let me know any more info you would need. I am attaching the restored AdobeXML.dll in a zip archive, if that could help.
AdobeXMP.zip