Dictionaries reflect common usage and shared understanding, they're not some infallible source. If the technology community started using the term "open source" in a different way, the dictionary would have to be updated to reflect the new meaning.
The definitions in a dictionary are for general purposes and not used, necessarily, in legal battles. There are legal definitions for things like Intellectual Property, Copyrights, Patents, and Trademarks. These definitions are what will be used when it comes to determining, legally, what open source is - not some wordsmith.
Noah Webster did use a strongly prescriptive approach when he wrote his dictionary. His decision to use alternate or simplified spellings of some words is still seen today as the differences between American English and British English. Linguistic prescription - the idea that there is a single "correct" language and other uses of language are somehow inferior or improper - was used to intentionally to give American English it's own identity.
Fortunately, most modern dictionaries recognize that languages evolve, using a descriptive approach, updating definitions when needed to reflect how words are actually used.
This definition seems suitable: https://www.merriam-webster.com/dictionary/open-source