You have to explicitly create a read-all policy for anon keys, and with no constraints, for people to get access to it.
The default is secure.
If you turn off RLS, there are warnings everywhere that the table is unsecured.
The author goes on to compare this with PocketBase, which he says you "have to go out of your way" to make insecure. You have to go out of your way with Supabase, as well!
I wonder if the author tested this? I do agree that some third party website builders who use supabase on the back end could have created insecure defaults, but that's not supabase's fault.