This isn't true as the article states earlier that Murder at Full Moon – has survived unseen in an archive ever since being rejected for publication in 1930.
As this was never published, how does copyright law affect it? Will the estate have no say in the matter when the novel becomes 96 years old in a few years?
I am incorrect: https://copyright.cornell.edu/publicdomain
It is lifetime of the author + 70 years.
[1] https://www.lib.purdue.edu/uco/CopyrightBasics/basics.html
Under modern copyright law, if it's a qualified work, and it's recorded, copyright begins then. No intent to publish or special marking or registration or other such ceremony is required. Marking and registration do have benefits in lawsuits about infringement, however.
For works created before the modern automatic copyright, other rules probably apply.
This was written before 1978, was not published before then, and not published before 2003 either, so it is protected by copyright for 70 years after Steinbeck's death: he died in 1968, so through 2038.
As to the hypothetical unpublished work that the publisher takes and publishes as their own: with newly-created work nowadays its easy: the work was copyrighted at "fixation," no need to publish or intend to publish. Before 1978, the author had state-level common-law copyright in unpublished works that would have applied.
They don't. It's currently in the Harry Ransom Center archives at UT. https://lithub.com/john-steinbeck-wrote-a-werewolf-murder-my...
But once the copyright runs out then the publisher can publish it regardless, and a viewer could quietly copy it and publish it, unless someone in the estate made altered copies for anyone viewing it, then the altered copies would have fresh copyright, confusing the whole issue.