The issue is not the radioactivity, it’s the power density. Per the article, this is ~24x smaller than an average phone battery, but can supply only 100uW.

I have a relatively conservative phone use, and on average, my phone uses 450mW. That means that you’d need 4500 of those batteries in your phone. But the battery would also need to cover the power usage peaks, which are multiple times higher than the average power consumption.