Tinymist uses even patch releases (e.g. 0.12.18) for regular releases – this would be equivalent to Typst 0.12 – and odd releases (e.g. 0.12.21) for pre-releases – this specific one uses 0.13 (or a Typst commit very close to it) but isn’t considered stable enough for Tinymist 0.13.0 yet.
So while Tinymist (necessarily) uses a fork of the Typst compiler to support its features, using a stable (even) release ensures you’re using the released version of Typst.
btw, I updated the title to bring it in line with the question guidelines:
Good titles are questions you would ask your friend about Typst.
I also added tags, as it makes your question easier to find.
Finally, if Eric’s response answered your initial question, marking it with a checkmark 
would help others find the answer more easily. Cheers! 