Connect includes a setting called "Blocked Instances includes comments", which will filter all comments from all users from any of your blocked instances.
I don't think Sync has this option... and I don't know whether it filters them or not.
If I had to guess it's a mix of people taking "I'm too lazy" at face value, people grossly underestimating how much work it takes to run and moderate your own instance, and people simply downvoting me for my original comment disliking the amount of tankies on lemmy
It's really not. Run it in Docker on your home PC. You don't need to moderate any users because you don't need to have any users on your instance. It quite literally is "just set it up once," defederate instances you don't like, and occasionally upgrade the software like you do any other app.
That being said, a few Lemmy apps now support blocking entire instances, so that IS more simple than it was even a few weeks ago.