That’s not at all what that topic was recommending. You should start your new forum on a new subdomain and leave the old one behind.
I think that’s right. If you really want the old forum and the new one on the same domain (which, again, seems like a horrible idea for the issues you have already outlined), You could conceivably have another domain that’s a CNAME for the old one and have Discourse link to those, then it would think that it was a different server and wouldn’t try to handle those links itself and when nginx got the link, it would either just serve the data from the other domain or redirect the browser to the right one.