Hi, the short answer is: most likely. The GATT treaty really did come into play for editions produced by the Muzgiz because they were produced by a government agency and were PD in Russia itself back in 1996. No NIEs were filed for the major editions, either, though a large number of them were filed for original works of composers like Prokofiev and Shostakovich, plus a fair number of arrangements and orchestrations. As far as we're concerned, you're free to post these at IMSLP.
There hasn't been a court case in the US covering music editions and the extent of their copyrightability, though cases like Feist v. Rural and Bridgeman v. Corel would appear to indicate that such editions fail to meet the necesaary "threshold or originality" to qualify for US copyright as a "derivative work." (I also expect that most music publishers aren't particularly eager to find out just how protectable such editions are, much preferring the present atmosphere of ambiguity.) Also note that many of the Muzgiz editions remain in print to this day with Kalmus/Alfred, Kalmus itself, Dover, etc.
Since IMSLP abides by Canadian law, which also requires that a work must meet a "threshold of originality" in order to qualify for copyright as an "adaptation", it's highly unlikely the Cui editions (or any urtext-type edition) are protected in Canada. IMSLP has adopted a voluntary policy on Urtext-style editions of not allowing anything published after 1989 in Russia, and a maximum of 25 years from publication for such editions published in the EU, including the former Warsaw pact countries. Italy, for example, allows a term of 20 years from publication, while Germany and the UK allow a 25 year term. The EU itself allows a maximum (not minimum) term of 30 years from publication. So, I think our voluntary policy is fairly reasonable as it stands. We also welcome new editions for posting under one of the Creative Commons licenses, of course.