This is their methodology:
Keep in mind this is not measuring Nostr to something else, it's measuring Nostr to itself over time, so while methodologies might be flawed trends over time can still be informative.
Growth is important in terms of ensuring survival. Decentralised protocols like Nostr do collapse. Secure scuttlebutt, which had 30,000 users spread across several clients at the peak and was arguably even more decentralised than Nostr, is no more. Devs need to feed families and take on other work, every morning something that used to work fine stops working, and a spiral sets in. Many such examples.
If a point of sustainability has been reached then I agree that measuring growth is not so important. But if that point has not yet been reached then often times the only alternative to growth is collapse, as seen time and time again. So growth in that context is just a synonym for survival.
I'm optimistic on Nostr, I think it's very much alive, and building for it makes sense. But it's definitely not immortal "just because".