You should have said: A large segment of Polish society, to which you've got no access due to your conspicuous foreignness. They'll never tell you lots of different things. However, here's a hint that may help. Ask Poles: 'Is it true that GW is dominated by people of Belarusian ancestry?' - and see what they say. That's all the 'evidence' you'll need.
Polonius, for the 25th time, please provide a single reliable source
for the following two claims :
what is behind that unusual concentration of people of non-Polish ancestry in a single newsroom and Agora boardroom?
That is only a partial list becuase many others are in denilal and hding behind good Polish-sounding names
Being foreign has nothing to do with it. If what you say has any merit at all, someone somewhere would have written about it.
The fact that you haven't provided a single source to back up those two claims suggests that you really did just make it up in your head and you're now desperately trying to change the topic.