Am I correct in saying that CMSImport doesn't update createDate, when reimporting using "Enable content updates"?

That's in any case the behavior I experienced.

I made an error during first import.

Running the fixed import again with content updates enabled showed that releaseDate got updated, but createDate remained the same. I had imported the same date from the source to both createDate and releaseDate, so I had the benefit of having the correct date in releaseDate. I could use this to my advantage.

I needed the createDate to be updated as well, so I used a sql script to help me there. I could make use of the releaseDate field to fix this. I share the script for future reference:

UPDATE N
SET createDate = releaseDate
FROM cmsContent C
INNER JOIN cmsContentType CT ON C.contentType = CT.nodeId
INNER JOIN umbracoNode N ON C.nodeId = N.id
INNER JOIN cmsDocument D ON D.nodeId = N.id
WHERE CT.alias = '[documentType]'
AND createDate <> releaseDate
AND (newest = 1 OR published = 1)