Open AI has an independent descriptive meaning as composite term. You would practically trademark a whole class of products, not only a brand name.
In contrast, open text is not descriptive in the sense of being a category of things. Therefore there is no risk that competitors would run into trademark issues by just describing their products.
Also, trademark decisions are always contextual to their time. Today’s meaning of ‘open’ in the context of software and data was not even coined in 1991, at that time people used ‘free software’ as term. Today I am not sure if ‘open text’ could still be trademarked.
>Open AI has an independent descriptive meaning as composite term.
See my edit. "Open Systems" also had an independent descriptive meaning. The phrase "open systems" was a very common generic phrase in 1990s when companies talking about POSIX compliance was a big deal. (E.g. Microsoft touted POSIX in Windows NT.)