Both things serve different purposes.

Imagine it this way, freenet needs electricity and servers to keep running. NOSTR messages can be printed in paper (handwritten even) and you'd still be able to verify it belongs to a specific person.

Basically freenet builds a network for communication (roads) but NOSTR is only about messages (cars) and doesn't really care about which road is using.

There is no centralization because there is no coordination. There is not even knowledge of what can be happening elsewhere because these messages might not even be using internet to be shared (e.g. radio or paper messages)

On the other side this is what makes it so powerful. You can download the full set of text messages from someone into your disk, that disk be found centuries later and digital archaeologists could easily read the contents because it is plain text.

So it isn't competing against freenet, it will use it very happily when available as option.