They knew, or should have known. They knew exactly what he had bought and in what quantity, and anyone who knew anything about radioactive material would have concluded it was safe, or if they had doubts, they would have sent maybe two people to go knock on his door and ask to look around.

This was someone or a small group inside the border force who didn't have a clue what they were doing, cocked up, tried to make a big showy scene of things, and then scrambled to save face after the actual experts clued them in that a) what he had was safe and b) was 100% legal to own. (note that he was prosecuted for something that the border force allowed through years before the sample they erroneously thought was a problem, and that was not illegal to own, only illegal under a very twisted interpretation of an obscure law to import).

They did know. It was well labelled and initially stopped at customs.

They asked the ordinary courier (without hazmat gear) to deliver it in person to help build a stronger case.

Details: https://www.youtube.com/watch?v=M0JGsSxBd2I

The hazmat crew was literally manufactured drama for a prosecutor (who somehow continues not to be named in this ridiculous case) to build a better case.

They stopped it at the border, then let an ordinary courier deliver it. Either they knew it's harmless or they're intentionally criminally negligent.