Being myself a full-time open source developer, I have seen several cases in which businesses are built around free software, either because multi-billion companies hire third party teams to build software for them under the conditions (imposed by the hired team) of open sourcing what is built, or by developing free software while offering high-end paid support for it, or even by wrapping the open source software in commercial enterprise-level, services-oriented applications.
So no, I do not think that it is reasonable to paint open sourcing is an equivalent to being stolen by big tech companies. It is just a particular way of doing things, originally based on certain principles and which sometimes succeeds as a business strategy and some other times does not, just like any other business strategy out there!
Having said that, I wish the best of luck to Marak and I hope they figure out the way to remake their life after what happened and continue to push on their awesome work on open source software while managing to make a living out it!