How should I interpret a promising preprint that was never published in a peer-reviewed journal?
There might be any number of reasons. You might try to contact the author(s) to get more information. But... (not all with the same likelihood)

They might have left academia for various reasons and not bothered. Is the CV also old? 

They might have incorporated the key ideas into another paper with a very different title. You search is then fruitless.

They might have discovered errors. 

Reviewers might have considered the results trivial.

Their attempts to publish might have been rejected by journals for other reasons. 

They might have changed sub-fields. (This one less likely, I think.)

But you should be wary, at least, of following up on unpublished work and, at least, be sure that you can verify the claims independently.