FFS this is a study. Its not trying to be clickbait, or lie to you. If its ethos is that it's just a correlation, then it's just a correlation. Would you rather not publish it?
This study truly is an association. It measures US and UK adults. You literally should take this at face value. However face value is just what's in the headline.
There's literally no way in a federated environment to prevent blocked accounts from interacting with your posts without making your blocked accounts list public
JPEG also supports lossless compression.