This is a really good explanation for how defederation works.
I understand your point that Beehaw defederating from two subs for moderation and user management seems like an extreme reaction. But it's one I kind of expected from them given Beehaw's philosophies as as an instance.
Their detailed posts about what Beehaw is always made it very clear to me they think carefully about how they run their space and the users they want to grant access to. They really prioritise making their instance a safe space for well-meaning discussion through their vetted registrations.
I'm not an admin. I'm not an experienced Lemmy user. I'm not someone who has had experience moderating and being an admin on several communities before. They have and I've also seen activity on the Lemmy repo from them showing they have dev experience too.
As you pointed out, the entire site of 12k users is currently managed by 4 people who seem to have quite a lot of experience managing communities. That's a big workload. I've been using both Beehaw and Kbin since Reddit's awful API changes to see how both places grow and so far I've found Beehaw to be a very enjoyable experience with a pretty high engagement rate. I usually get hella upvotes and replies to anything I say. It does feel like a pretty active, close-knit place of well-meaning people even at this early stage. I think they're running Beehaw pretty well so far. Kbin is very solid too, but Beehaw I've found tends to have a deeper level of engagement and longer, more in-depth post styles that I prefer.
I know any instances with open registration could hop in and contribute to Beehaw, so this issue they have of not being able to vet and control users isn't unique to those two instances. But given so far the place to me as a user still feels the same as when I joined a few days ago more or less, I'm going to take them at their word that they're getting an influx of activity that isn't a particularly good fit for Beehaw for now. There's a lot of instances that could defederate from. 2 is not a huge number so far. Plus they did explicitly say at the end this is not a permanent decision, they may very well change their minds later on.
So personally, I respect and understand Beehaw's decision at this moment. Lets give things time and see how things develop. It's definitely a temporary, broad axe to cutting an apple type solution to their troll problem - which may very well continue as Lemmy gets more popularity as a platform overall - but I think they want to be specific about who they pull into their moderation team to ensure the vibe of Beehaw is maintained. Lets give it some time to see what happens.