I was kindly pointed to the debhelper generic option --remaining-packages. I can imagine that the "real" dh_strip calls some debhelper functions to honour this option (it uses $dh{DOPACKAGES}), but I guess the dh_strip wrapper in pkg_create_debug does not so it will do its ddebs work on all packages before handing over to the real dh_strip.
I was kindly pointed to the debhelper generic option --remaining- packages. I can imagine that the "real" dh_strip calls some debhelper functions to honour this option (it uses $dh{DOPACKAGES}), but I guess the dh_strip wrapper in pkg_create_debug does not so it will do its ddebs work on all packages before handing over to the real dh_strip.