No, this is not an illegal remake and in fact, this is an enhanced version which is based off the original game mechanics, not a direct copy. The original intention was to test pygame's performance speed for arcade games.
I don't have assets to rip off, because I don't have any Defender arcade machine. If you see some of the screenshots, you will see that all of the images are different than the original - entirely made using primitive drawing methods like line, circle, etc. Even the terrain is different and randomly generated each time.
The code is made in python using pygame unlike the original game which used Assembly.
This game is based on the same mechanics as the Defender, but the assets are also my own. If you hear the sound effects used, you will realise that they are slightly similar but overall different from the original.
With that being said, this is a purely non-profit and non-commercial remake. Also, it uses different features like fullscreen toggle, sounds settings, custom key-mapping, and most importantly an online leaderboard, which are entirely different than the original game. So, you can say this game is based off the original game, but not a direct copy of it.
Hope this clears any doubts. Do tell me if you have more questions :)