A Public Benefit Corporation has a mission for the public good, but can take investments in the way a nonprofit cannot. This also means it can move much faster. Given the influx of users to Bluesky, taking investments this way may have been the only load handling route available this fast.
Again, this is all tuned to "What is Bluesky trying to build?"
Bluesky might not be a good "decentralized Twitter replacement", but it is a good "Twitter replacement" with the possibility of "credible exit"