Regarding this:
Release notes for 13.8 includes the following text:
Directory Opus 13.8:
That suggested to me that one of the fields added was EXIF-CreateDate.
But no. That was not the field that was added.
The new Date/Time Created field (unique to Opus, I believe) is a combination of two fields: XMP-DateCreated and IPTC DateCreated.
IPTC and XMP fields are not EXIF fields:
https://exiftool.org/TagNames/EXIF.html
...but some applications look at certain EXIF fields and then add matching XMP and/or IPTC metadata. For example, when Lightroom Classic reads EXIF-DateTimeOriginal, it adds an XMP and an IPTC field with a date and time matching the EXIF-DateTimeOriginal.