Patent offices, at least in other countries, haven't accepted perpetual motion machines (or time travel devices) for some time. It's my understanding that the US no longer grants them either and that part of the justification in so doing is that there is a-priori knowledge on the part of the patent office that a working invention does not exist.
It's also been my understanding that you have to have prototyped at least the key parts of the invention even if you have not prototyped the invention as a whole. There has to be a minimal implementation. Certainly in companies I've worked at in the US where they were actively patenting inventions, that was a requirement the patent lawyer stated had to be fulfilled.
It would seem Groklaw has the same understanding, so if a patent lawyer and a paralegal come to the same conclusion, I can only conclude that that is the legal requirement. Doesn't make it the requirement in practice, merely the requirement as set down by law.
The US patent office, again according to my understanding, does NOT evaluate patents but waits for others to do so on their behalf. If this is correct, there may be many technically illegal patents in the books. If the system relies 100% on challenges, the law merely describes what can be enforced rather than what can be filed.