I wonder what it means for exobiology, is this true only in Earth conditions or in general, i.e. can we expect life elsewhere to use the same 20 amino acids? Also, if not, how toxic/dangerous would that life be for humans?
About life in other planets, nobody knows, so it's guessing time:
The 20 amino acids are some of the ones that get formed spontaneously in the prebiotic conditions, so it's probably that they exist in other planets. In some proteins, the amino acids are modified after the protein is produced, so a few additional amino acids may be a nice feature. Some amino acids of the proteins are quite similar and perhaps we can live without them. So will independent life in other planets:
* use the same 20 amino acids: Probably no. Unofficially: No way.
* use the exactly 20 amino acids: Probably no. It's a hard question, there are some paper that try to justify the number 20 or a similar number, but I'm not convinced. I guess that something between 15 and 30 amino acids.
* Use some set of amino acid similar to our 20 amino acids: Probably. I guess yes.
* Use the same L variants as us, or use the specular D variants: Let's say 50%.
* Use proteins at all: Another hard guess, I'd say yes. Proteins are very versatile and easy to build in different configurations. It's hard to guess a replacement.