In most cases you are right, he doesn't list the release dates, especially for the 4000 series. But look at
Cupol 9001 Mary's boogie / Instrumental / Mary Lou Williams, piano and her boyfriends / N.Y. 04.1944 - utg. 08.1951 / *
Actually, even for Cupol 9011 Svänga-Benga does not provide a release date. So who knows if that record was ever released on 78 rpm. The strange thing is, that others in the same numerical sequence did get released, usually in 1952.
It certainly would explain it. Rather odd that she had four Cupol 78s, and according to that site, only the first (9011) was actually released. I would guess that as they’re all in the same range (9011-9014) that Cupol scheduled them all with their release list for that period, then something happened after they’d assigned the numbers. Maybe the licensing deal with Polygon fell through, as I don’t think Cupol issued many other 78s from them after this - perhaps one or two based on the discography. As they were unreleased it might explain why no copies have been seen by collectors or on auction sites. Perhaps Petula’s Cupol 78s should be listed as Unreleased, bar the first (which also hasn’t shown up, but maybe it’s just very scarce). Four 78s in catalogue number sequence often also suggests an album set, but I don’t know how popular those were in Sweden.
Now I wonder why Svänga-Benga lists Cupol 9011-9014 as "not released." It would explain, of course, why none of these records is illustrated by a label scan. ;-)
This is listed as recorded in London, Nov. 1950, but as "Ej. utg." (note sure what Ej. means) in http://78-varvare.atspace.cc/cupol.htm. Nearby numbers seem to have been released in 1952, not 1950, so I have changed the date here on on the following two Petula Clark recordings on Cupol, which were not even recorded before 1951.