That source (published by the developers) is a primary source. Encyclopedias, including Wikipedia, are tetriary sources that are based on secondary sources and not directly on primary sources.
In other words, unless there are reliable secondary sources to base the article on, it is considered original research and is not a good fit for Wikipedia.
I guess part of the reason that new contributors feel bad is confusion about the goals and nature of Wikipedia itself.
It's not just goals and nature. It's rules that can be pretty silly and unhelpful when they're taken to extremes rather than applied sensibly. In this case you have documentation that can't be referenced in a Wikipedia article. Yet, if someone wrote a blog post that liberally quoted that documentation, that would probably be an acceptable source.
The basic ideas of not doing original research or relying on primary sources are fine. But writing just about any article requires synthesizing multiple sources to some degree. Rules are one thing. Saying that the documentation is not a suitable source for information about a file format is something else.
Wikipedia does consider the documentation suitable for supporting information in the article, but not suitable for establishing the topic's notability.
The article needs independent sources (as in, secondary sources that are financially and editorially independent of the company who develops the .mix file type) to show that the topic warrants an article.
If an article has enough sources cited to show notability, primary sources like documentation pages can be used. If notability is not shown, then the topic doesn't meet Wikipedia's inclusion criteria and the content of the article is moot.
Without this requirement, any company would be able to publish promotional articles on all of its products, and exclusively use its own web pages as citations. Wikipedia's notability guidelines are in place to prevent spam and to ensure that topics only get articles if they can be written about in a neutral way.
In other words, unless there are reliable secondary sources to base the article on, it is considered original research and is not a good fit for Wikipedia.
I guess part of the reason that new contributors feel bad is confusion about the goals and nature of Wikipedia itself.