This is clause 1d) of the agreement. One way or another, this copyright hand-over never happened, and Amiga Inc. is still registered at the US copyright office. See above for the link.
The US hasn't required registration since it joined the Berne convention in 1989 (not requiring registration is one of the requirements of signing up to the Berne convention). You CAN register with the copyright office if you want everyone to know who to contact. However in terms of copyright ownership, it's largely irrelevant. You don't need to have registered your copyright to issue a DMCA takedown. Supposedly to get awarded damages in court you need to have it registered, but that goes against the Berne convention (especially for owners outside the US).
Neither Hyperion or Cloanto are US companies. So they are unlikely to worry too much what the US copyright office or supreme courts say especially as I don't think Hyperion or Cloanto are going to take anyone to court in the US to get damages from them, while their agreements and previous case rulings are enough to protect themselves in court.
Amino were willing to relinquish most of their IP for $1?
It's hard to transfer legal ownership without consideration.
https://en.wikipedia.org/wiki/Consideration_under_American_lawIf two parties are suing each other and want to escape from the court case then they will often do deals like this where property changes hands. A famous example is DEC suing Intel and when they settled in 1997 and Intel agreed to buy StrongARM from DEC for $700 million
https://www.cnet.com/news/intel-digital-settle-suit/. The money was essentially damages for the technology that Intel stole, but DEC also got to offload StrongARM as they weren't able to make money out of it. Intel kept it going for a while, but in 2006 they sold it to marvell for $600 million.
Amiga Inc agreed to sell it to a specific person for as part of a bigger deal, a random stranger wouldn't have been able to jump in and offer $2.
