I understand where you're coming from, but "open source" does not mean "I can read the source". If that were the definition, then Windows 2000[1] is open source.
That qualifies as Open Source under any reasonable definition of the word [1].
So being Open Source is no argument for or against either Adblock Plus or ublock origin, because both are.
The OSI definition of "open source" is not the only definition. I would argue that most people see "open source" as software where the source is "visible" (no data to back that up).
Then you have a gazillion of "Open source licenses", that can be less "free" to more "free", like BSD, GNU, MIT, Apache, the free source license, ...
My definition of open source is software where I can modify the source and use the resulting modified software without unnecessary roadblocks or fear of legal reprisal.
That qualifies as Open Source under any reasonable definition of the word [1]. So being Open Source is no argument for or against either Adblock Plus or ublock origin, because both are.
[1] https://opensource.org/licenses/gpl-license