So how did they replicate, and what was the selection process?
They find out stuff that are too soluble can't fold well because they prefer water over other amino acid. And that seems to exert a selection pressure, ending up with a specific set of amino acids that life depends on.
That's a teleological fallacy. Evolution doesn't have a destination, therefore you can't assume the evolution criteria from the end result only. Imagine looking at all of life, adding a 'resulting in human life' criteria and therefore completely failing to understand the evolution of avians.
If the authors want to argue amino acids 'evolved', they need criteria that make sense for amino acids in early Earth environment and not assume the result.
The context is for life on earth. All life on earth depends on proteins that are (mostly) soluble in water and almost all protein requires folding to be functional. It is a valid hypothesis to say solubility and foldability are necessary criteria. Then they tested it and found based on these criteria, the current set of amino acids is favored over other sets. Look like good science to me.
Necessary criteria for life, yes. Is it neccesary for evolution? The article's argument is that amino acids 'evolved'. Evolution needs a selection mechanism, and the article does not explain why evolution would favour solubility and foldability, so the article* doesn't come close to proving evolution happened much less explaining its details. Perhaps some other unexplained chemical process favoured these acids, or it was an accident.
It's like looking at current status of biosphere, and concluding that life optimizes for insectoids because insects have the largest biomass. It's just an argument that assumes the result.
* Perhaps the article is bad reporting and the reported study had an actual argument for this.
The term "evolution" was used very loosely in the salon article. By your standard it would be more appropriate to call the process "selection". The amino acids never changes or evolved to become anything different, just that some were chosen to become the building blocks of life while others were ignored. The reasons for this selection were likely the two criteria mentioned above and that was backed up by the main study.
You will notice I never used "evolution" to describe the process in my posts. I skipped the salon article entirely and only used the link they provided to read the real study so I didn't notice what they said in the salon page. Anyway, it is just some semantic stuff.
[0] "So by testing which libraries had the best solubility and structure, the researchers could conclude that proteins were evolving and driving natural selection before even forming living things."
[1] https://blog.devgenius.io/origins-of-life-building-a-hypercy...