If the li-ion cell do not have any protection circuitry, it could be dangerous as they said. I think most of the cell has internal protection circuit. If you break the cellphone's battery, you will get that one chip inside. Maybe that's protecting in your case.
My two experience:
- Once battery's voltage was 0V. I broke the cell and removed the chip. Then measured the original output pole of the cell and it was upto 2V. So we can say that chip was disconnecting the cell to make safe us. Charging a cell having voltage lower than minimum in Li-ion can explode.
-Once bettery was fully discharged mistakenly and cellphone was not detecting it. So I charged it using cellphone's 5V power adapter directly. I forgot to disconnect charging and it was being charged whole night! I though it's about to explode because of OVER charging. When I put the battery in cellphone, charging is just 10%. It means the internal chip disconnected the charging process after few minute when it get something wrong charging voltage.
But be careful, the tiny chip always might not be able to protect our precious life.