They might not know who you as an individual poster are, if you practise good op-sec, but they certainly know which relays you are using.
Discussion
But if they don't know me why would knowing the relays help them? I'll just use different relays.
It becomes a game of whack-a-mole: but if you as a poster or sender of messages don’t know that some of the relays that you post to are now blocking you (because why would they tell you?) then you basically become shadow banned.
Until your followers realise they haven’t heard from you in a while and start looking for you and start following one or more of the relays that you are still connected to.
That is why having lots of relays in lots of different jurisdictions is very helpful agains government interference.