I read the text: it's license hermenuetics at best and FUD at worst. Has there been a single instance in recorded history of the author of a public domain work trying to enforce usage, modification, or distribution permissions. Sure, you can point to theoretical variation in the precise semantics of the public domain in various jurisdictions, but it feels like a bar exam puzzle, not a real world practical concern. In the real world, you can safely do whatever you want with public domain software. It counts as free software. That half the planet nowadays uses SQLite and treats it as free software is testament to this reality. Obscure license pedanticism just doesn't inform the choices of anyone actually building.
Open source and Free software have different philosophies, but in practice they are essentially the same. You are thinking about copyleft vs non-copyleft. BSD, MIT, CC0 are all Free Software licenses but not copyleft.
You’re making the common mistake of confusing the copyleft vs. permissive distinction with the free software vs. open source distinction.
GPL is copyleft. MIT, BSD etc. are permissive. But all of those are both free software and open source, which are essentially synonyms.
The reason so many people get confused by this is that some of the people who prefer copyleft licenses (notably the FSF) also tend to prefer the term “free software”, for philosophical reasons.
It might seem really unlikely any acquirer would ever sue, but if your big company has compliance auditors they will need to see something in black and white.
