It's absolutely an improvement! I don't think that's being disputed, it's more that using the audio fed to the DSLR instead of the tracks recorded directly on your F8n means choosing to add noise (and potentially some coloration of the sound) due to the DSLR pre-amps. While the audio from the F8n fed to the DSLR will still be a vast improvement from a Video Mic, it's in the nature of sound folks to want to recommend the workflow that will result in the absolutely best, crispiest sound you can get. 😄
I'd also add that, if you're interested in the entire process of filmmaking and audio through to post-production, editing footage without syncing the ISO tracks from your mixer before you start cutting can have a cascade of sometimes unfortunate effects on the post audio process. For your post sound editor and mixer, having more flexibility in how they build the audio world of a film means you get better results. Sometimes a mix track sent to a camera is perfect and able to be used in the final mix. Other times, and especially in narrative, the sound folks need to get in and edit or remove things in the individual isolated tracks (boom, lav, plant mics, etc) to build a quality mix.
It may be that you're doing it all yourself right now, but planning for the future and building good workflows that will scale up to when you are able to bring in more people to work on your films is never a bad thing. Dual system (camera and audio recorded separate) is pretty common, so learning those workflows now will only make things easier for you as your work and films grow.