A few guys claim that the prisoners from a certain prison might escape through the tunnel that was dug up a long time ago underneath the prison.

One of them says: From what we know about this prison, there's no way that escape is that serious of an issue.

I take it to mean that the authorities of the prison don't take the possibility of escape from that prison seriously? What do you think?
I take it to mean that escape is easy enough without bothering to find the tunnel.
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Hmm, that's a bit strange I guess.

A bit wider context. It's about a supermax prison.

One says: Prisoners could escape that way.

The other one replies: From what we've been told about this supermax prison I think there's no way that escape is that serious of an issue. Every prisoner is kept segregated, and then shackled and searched when they need to take a leak. There are almost as many guards as cons, and they only get one hour out of their cells a day.

Do you think it still means that probably the escape is easy?

Maybe they guards don't care about it because they know no one will manage to do it?
No, since you have now included the word 'supermax'. The phrase means that probably any escape is too difficult. The seriousness is that of the prisoners, not the guards. There is no reason to think seriously about escaping because escape is impossible.
Now it's clear. Thanks.
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I think they are saying the tunnel isn't going to lead the prisoners far away from the prison and they probably have the other end guarded. That's why they don't seem concerned.