If you look at how Apple detects contraband imagery, they hash every image that gets uploaded into the photos app. Those hashes are transmitted to servers that compare them to hashes of known contraband.
A similar system could theoretically be used for STL files. So it isn't about detecting exact shapes, it's about preventing printing of STL files that are already known to be dangerous. This would make it harder to illegally manufacture parts for weapons because it would make it much harder to share designs. If you didn't have the knowledge or skill to design a reliable FCU, you would have to find a design someone with that knowledge and skill created - which the printer could theoretically detect with a cryptographic signature.
As the original author of the post pointed out though, this could and would be bypassed by actual criminals. As with most things like this, it's probably impossible to prevent entirely, only to make it more difficult.
