For example, this theory sti doesn't answer the question how they came into existence in the first place. It's certainly not obviously better than assuming they suddenly appeared on another planet and then transported to earth..
Not sure what you're getting at WRT amino acids. Amino acids form due to the particular chemistry of carbon (what we call "organic chemistry"[1]), and they do so wherever both energy and the building blocks of amino acids exist. No planet required.
This was shown pretty well by the Miller-Urey experiments[2] as well as the discovery of amino acids in interstellar molecular clouds[0].
So...Amino acids can (and do) form wherever the raw materials and energy exist. That's the "how".
[0] https://physicsworld.com/a/amino-acid-detected-in-space/
[1] https://en.wikipedia.org/wiki/Organic_chemistry
[2] https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment
Space is really fucking empty. Seems less likely than throwing a rock out of a rocket on the way to mars and hitting a random bucket, without looking. Add to that that it would require life having evolved while the universe was younger and different to the only state of the universe that we know to support life.
Is it impossible. No. But at that point the theory has as much explanatory power as creationism, i.e. none.
You may not have heard about it, as the paper describing the experiment was only published 69 years ago.