The license seems a bit iffy. The blog says it's "open source", but the license is a modified version of the SSPL, which is not recognized as an open source license in the customary sense of the term.
They also don't say explicitly what modifications they made compared to the SSPL. Some diffing shows that the changes consist of:
1. Search/replace of license and publisher name.
2. Renumbering sections with 1-based indexing.
3. Adding the words "commercially or competitively" to section 13 (which is renumbered to 14).
In the process of doing this, they seem to have accidentally replaced the publisher name with the license name in one place ("any version ever published by the MoonBit Public Source License") and introduced an off-by-one error where section 18 (which used to be 17) refers to sections 15 and 16.
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