As Andreas notes, the creation property is read only for scripting, and as you’ve noted, the standard non-scripting import processes create “now” and so the timestamps reflect that.
There is a way to do it, but it needs a bit of work to do:
Now, it would be possible on the Mac to build an automation to convert a file into a Drafts json file ready for import. Pretty much any scripting language is going to let you read file meta data (creation date, etc.) and write it out to a text file with the content and some standard text to create the JSON. The ease of doing so just depends on what tools or scripting languages you are comfortable working with, and how well suited they are to this sort of thing.
Hope that helps.