Apparently this is rare? Mushroom island next to land in 1.17.1

I watched a youtube video about different minecraft anomalies and it mentioned that mushroom islands being close to land is very rare. I immediately remembered this place in my survival world, mushroom island right next to a big island, it also has small patches of grass on it. How rare is it actually?
(I did dig up a few blocks on the shore, but the shape of the island is the same as when it was generated)
(Also I found this seed on a random website with cool seeds 4 years ago lol)