This is a good effort and is interesting to say the least, however, I do have a couple problems with it.
The game really lacks any sort of inspiration. I guess the original playstyle for Defender was too hard and you sort of gave up and used the arrow keys, sacrificing what made Defender fun and making it an autoscroller. No smartbombs or Hyperspace to be found either.
Your planet doesn't explode and send you to space when all your humanoids die; making the only consequence for letting humanoids die is losing points. This wasn't enough to stop me from just letting them get captured and shooting the mutants for more points.
Having the Williams logo as the thumbnail led me to expect either a sub-par Defender remake or an improved reimagination of the game. I had a good laugh, but in the end, I was a bit disappointed.
For the record, a more true Defender experience (while a bit limited to what such an elementary engine can perform,) has been made in MIT Scratch Defender in Scratch but as I see things right now, this isn't the aim for the current release.