SUMMARY
Dolphin generates thumbnails for folders when folder thumbnails area deactivated and previews are enabled. The issue seems to lie in KIO or the plugin itself.

STEPS TO REPRODUCE
1. Disable all thumbnails in Settings=>Interface=>Previews 
2. View Settings in Compact mode with Previews enabled

OBSERVED RESULT
Folder thumbnails being created in .cache/thumbnails/

EXPECTED RESULT
No thumbnails being created.

SOFTWARE/OS VERSIONS
Operating System: Gentoo Linux 2.18
KDE Plasma Version: 6.5.0
KDE Frameworks Version: 6.19.0
Qt Version: 6.9.3

ADDITIONAL INFORMATION
This is a regression, I can't really pinpoint the exact version but thumbnails were not created before I last updated(~last 2 months).

I did a bit of debugging, as far as I could:
-Dolphin calls filePreview with an empty(not nullptr) enabledPLugins.
-FilePreviewJob is called with an empty mimeMap
-The thumbnail is generated in slotStatFile
-m_plugin is never initialized and is the default one(I'm not sure this is expected, it seems weird to me)
-KIO::TransferJob "thumbnail" schema is called with this( https://github.com/KDE/kio/blob/master/src/gui/filepreviewjob.cpp#L594 ):
kf.kio.gui: mimeType: "inode/directory"
kf.kio.gui: width "0"
kf.kio.gui: height "0"
kf.kio.gui: plugin ""
kf.kio.gui: enabledPlugins ""

And it ends up generating a thumbnail.