> from the main page of my Miniflux instance, and then put /feeds on the end
This case is relative to a single baseURL. The poster was proposing to have a bookmark relative to the URL you're currently browsing from, so that you could click "/feeds" from any Miniflux instance you're currently visiting.
You can do this it's always /feeds. I suppose if someone really wanted to change that, they probably could. I used to use the hosted bersion, and it was /feeds there too. Although I have to point out that unless you have access to the instance, you probably won't be able to get to that page without signing in anyway. Unless someone decided to make the entire thing public, for some reason. You can do this with tweets even, you just have to have the URL of the tweet.
Please let me know if you can't access it.