They couldn’t reach out to specific developers, but what if they just sent out a tweet that said “Hey developers, we’re trying to develop new features for embedded streams. How do you use embedded streams? Would it ruin your users’ experience if there were intrusive popups over the center of the content?”
They also could’ve added the feature in a way that you’re opted in by default, but let the developers opt out of it, and then track how many devs opt out to determine if anyone likes the feature, or to indicate that your research missed a popular use case for embeds that this feature is damaging.