The title more or less says it all. Having trouble figuring out how to redirect a Sitecore address to an external website URL, or if that's something that the SXA redirect map is capable of doing
No, Redirect Map can only redirect to other items on a current site (it will try to append to a URL string that you define in the mapping).
Redirect Map + Redirect Item
Although it is not a perfect solution you could redirect to an existing Redirect Item (Redirect a URL) and define external URL there.
One important thing about Redirect Map. It will match your URL only if Sitecore is unable to resolve the item (404), otherwise, it will return the item.
Wildcard Item / Redirect Item
This is an interesting solution but it will not work for every case. You can try to combine wildcards feature with Redirect Item
This will match every URL that cannot be resolved on a first level and redirect to google