It seems to be true.
Node 54 for instance representing the major city Novosibirsk has no place id and thus the nominatim node id search returns nothing. 
I suppose what I need to do for such cases is search by name and area and hope to find other entities (ways/relations) that do have place ids; this is somewhat annoying since then I have to disambiguate the potential multiple results; the nice thing about a node lookup is the node serves as uid.
It seems somewhat more involved than necessary; is there some reason not every node tagged ‘city’ gets the honor of a place id (or alternately why nominatim will not return anything but items with place id?)
Now that i think of it I suppose in my original osmfilter run I should find not only nodes but also ways and relations tagged as city , and perhaps of those at least one will have a place id and thus be found in nominatim.
In any case thanks for your help, this has been useful.
