Whether or not the signature is genuine, it isn't impossible that it was made from a round, domed, paperweight that was then cut down. If you took a standard sulphide pw and cut it down on a diamond saw, then ground and polished it, you would end up with something like this. However, if you intended to make it look like this, while hot you would push it into a hexagonal mould and when cold you would grind out the chill marks and re-polish.
While it would certainly be easier to grind six sides then five, you only need to look at some of the overlaid Baccarat and St Louis paperweights with five lenses in the sides (and one on top) to see that pentagonals are not impossible.
As to whether the accuracy of this grinding and polishing is a huge achievement, however, needs to be taken in the context of how large a factory or studio produces such work. If the same person made the paperweight from start to finish, then that would be unusual in this context and perhaps quite an achievement.
There has been some discussion recently of the superb paperweights produced by Allan Scott with cutting by Martin Murray. I hope they will forgive me if I use them as slightly more than hypothetical examples! In a large organisation people become specialists in certain aspects of the glass making process. Allan is a specialist lampworker, while Martin is a master cutter.
If Allan asked Martin to grind and polish a paperweight to this level, he could probably do it with his eyes shut, and it would take less than an hour! However, I'm not sure how good Martin's lampwork is, and I suspect Allan's cold work is about the same standard, so if Allan had to do it himself it might take a few attempts and a bit longer to get it right!