It is open source in the sense that the source code is available for review. It's on the MIT License that they migrated to, to prevent other companies legally reusing their code in commercial competi...
Being able to view code doesnt necessarily equate to open source. Proton and Telegram are 'open source' in that sense, yet nobody knows or can know what is really running. Andoid is also 'open source' yet we all know that nobody knows whats really running. They fucked up, whether intentional or not. Big time. I dont understand why so many are making excuses for them.
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