The reason why he asked this was because there was no sign of any battle or resistance in the prison.
If someone had soul power or skills, then they should be able to destroy the prison quite easily. But in contrast to that, the iron bars that stood even now seemed perfectly fine, only a few scratches could be seen on them.
When Leon said that one could even force someone to sacrifice their beast souls, Leo understood that all the criminals that were present here had their beast souls taken away.
Only by doing that, they would be stuck at a certain level while losing all their skills.
But then again, they would still have a lot of soul power in their body - but if they couldn't channel it via skills, then it was actually useless.
Skills were just methods to be able to channel soul power out of one's body. If one loses all their skills, they would barely be able to use any of their soul power.