The title page of the Internet Archive asserts that the book was published by Marian Press, and the Marian Fathers of the Immaculate Conception assert copyright: https://marian.org/house/copyright.
Searching at https://www.copyright.gov/public-records/
I find a copyright registration record for the 3rd edition:
Registration Number / Date: TX0005274546 / 2000-09-01
Title: Diary : Divine mercy in my soul / Saint Maria Faustina Kowalska.
Edition: 3rd ed. / with revisions.
Imprint: Stockbridge, MA : Marians of the Immaculate Conception, 2000.
Description: 699 p.
Language: Original Polish version prev. copyrighted 1981 English-language version first pub. 1987.
The search I did returned 10,000 records: someone with more interest could do a better search for other editions.
It is certainly suspicious that Marion Press did not include any publication date, or translation date, or registration claim, for the copy found at the Internet Archive, leading to the suspicion that it was a re-publication of an existing public-domain document, but they aren't required to do that: it just makes it more difficult to claim damages.
Not claiming copyright doesn't make a document Public Domain: it may indicate that, at the time of publication, the publisher wasn't interested in making copyright claims.
Copyright law in the USA was different before 1978. That doesn't seem to have any relivence here.
