This is why you need to do these kind of things with a migration script and why it wouldn’t make sense for the WP Discourse plugin to let you force the post created_at date to match the Wordpress post date.
If I added that as an option people would attempt to do the kind of operation you’re doing, find it doesn’t work (because of things like the bump date) and then come here to ask for support with what is really a migration 
I say this in kindness, as I do understand what you’re trying to do.
The solution is to handle it properly, with a migration script. The plugin is not designed for bulk operations. Trying to make it work for bulk operations will always be more trouble than its worth.
For the normal use of the plugin, the Wordpress date does match the Discourse date, because the publication to Discourse occurs at the time of publication of the original post.