Nothing here addresses campaign managers/advertisers manipulating speech.
Can they threaten campaign positions for content that is not actually dangerous to anyone, just like the OP?
Yes, of course

"If you post about Kitchenaid mixers again, I'll stop paying you" is a perfectly fine deal. Take it or leave it.
Would that not cause people to compromise their honesty?
Did the OP really go out of their way to recommend Sparrow, or did they just participate in normal conversation?
If I have a signature for Bitcoin Core and someone asks "what is everyone's favourite wallet?"
Does that mean I have to say Bitcoin Core even though my honest preference is Electrum?
If I say Electrum can Bitcoin Core kick me from the campaign rightfully?
If I say Bitcoin Core instead of Electrum just to keep my signature, isn't there a big problem with that?
These are a lot of questions. I suppose the main one is: Does signing up to a signature campaign now mean that we have to be mindful (or have to compromise) our honest opinions?