The decay chain to radon is slow, but the ore itself already has plenty of the intermediate products, including significant amounts of the immediate precursor radium, as its been sitting around for billions of years. AFAIU most of the measurable radiation from uranium ore comes from radium decay, so, yes, I assume what he was detecting was the decay chain starting from radium to radon.

The half-life of radon is very short (a few days), but that's part of what makes it so dangerous, along with being a gas and the fact that radon decay is itself highly energetic regardless of the half-life.

You can see the decay chain here: https://en.wikipedia.org/wiki/Uranium-238#Radium_series_(or_... The whole radon decay chain (radon->polonium->lead) happens quickly and is the most energetic part of the uranium decay chain. And because you can easily inhale radon, it can all occur inside your lungs where even alpha particles (especially alpha particles, actually) can damage cells. Basically, a lot of energy released in a very short amount of time within unprotected tissue. So while uranium ore may emit only a tiny amount of radon, radon itself is nasty enough that you can inadvertently go from harmless to potentially harmful by storing more than a handful of uranium ore in a confined space.

I'm not doing the actual science justice and perhaps mincing terminology (not my field), but that decay table should speak for itself.