It isn't the case.
The first problem was actual trolls using were using lemmy.world's open and automated registration (beehaw makes you write why you want to join and manually approves registrations based on that) to troll Beehaw.
The second problem was that the moderation tools aren't mature enough yet to deal with problem one with anything between manually banning every troll (which will immediately come back by creating another lemmy.world account) and total defederation from the instances most of those trolls are coming from.
Because Beehaw's mission statement is to be a safe space, it was decided to go with the defederation option.
However, the defederation isn't planned to be permanent. Improvements in mod tools and/or maturing of communities are said to be reasons to rederate again.