To save anyone else from losing time on this bullshit:
They’re scared of their FOSS fork being forked. The rest of the article is just an attempt to make them sympathetic, and muddy the waters. That’s why GPL > BSD
A new project based on our code might implement features that are fundamentally in opposition to our ethics (e.g., damaging to privacy, human rights or to the environment). Even though we would not be associated with the project in any way, it can deeply affect how people see Vivaldi (and how we see ourselves), damaging a reputation we have taken pains to earn.