Seems like a good idea to check for and try any export tools before adding content to a system. Furthermore if the site provides the export tool there's no guarantee they wont take it away or modify it.
The best approach is to keep your source of truth out of the Medium platform from the start. You can use the "import post" functionality to tag your original off-Medium post as the canonical one, which gives you the best of both worlds: Medium discoverability plus full control over your content. https://help.medium.com/hc/en-us/articles/214550207-Import-a...