Yes, it is really adsb.im image because I downloaded it today morning from adsb.im official site.
Using same downloaded image (which gave trouble when used with Raspberry Pi Imager), I used Win32DiskImager, which does NOT have any option for user to make any settings. This improved the situation drastically. Now http://adsb-feeder.local did bring the “Basic Setup” page. However http://192.168.12.21 still continues to display Flightaware status page.
Question:
Why user name & password is not allowed in this image? This makes the image behave like restrictive Apple’s OSX & iOS, and not like a fully permissive Linux OS.
Sorry, above question / statement was due to lack of knowledge about “System Management”.
Now I found that I can ssh and login as root, and the root password is displayed by clicking the “SHOW PASSWORD” button under “System Management”
EDIT:
Clearing Browser cache (Ctrl + F5), replaced Flightaware satus page at http://192.168.12.21 by “Basic Setup” page 
