I think your suspicion that the date of the sidecar files plays a role is correct. The manual states (bolding mine):

Look for updated xmp files on startup

Check file modification times of all XMP files on startup to find out if any got updated in the meantime by some other software. If updated XMP files are found a menu opens for the user to decide which of the XMP files to be reloaded – replacing darktable's database entries by the XMP file contents – and which of the XMP files to be overwritten by darktable's database. Activating this option also causes darktable to check for text sidecar files that have been added after import time – see option “overlay txt sidecar over zoomed images” in Section 8.1, “GUI options” (default off).

So it seems you need to have XMP files that have a modification time that is later than the last database modification time.

Roughly the steps I would have in mind would be:

1. Create a copy of +/- 100 of your photos and XMP files into a separate folder
2. Import the copies into darktable
3. Set the "Look for updated xmp files on startup" options in darktable.
4. Close darktable.
5. Loop over your XMP files and touch them to update the modification time only (hence the -m option):

for file in $(find . -name "*.xmp")
do
    touch -m $file
done

6. Open darktable. I expect it now to replace the database entries by the XMP file contents.