It kills as a chemical, but is detectable as a radioactive substance. Using it means you have access to a nuclear reactor and extensive infrastructure. It is a weapon that could only have come from a state actor, one with an extensive nuclear program and a willingness to weaponize material from that program. That is a very short list. Had they wanted to remain anonymous they could have used any of a dozen different methods that could not be so easily traced. They weren't trying for a knife in the dark.

Polonium can be made in a cyclotron, and AFAIK they are quite common in hospitals. I'm afraid this is very much in reach of a non state actor...

Atoms of polonium can indeed be manufactured in a cyclotron, but unless your cyclotron is the size of a building it would take years to gather enough to conduct such a poisoning. Usable quantities are only ever made inside nuclear reactors.