@krzyk2: Hmm, that's a fairly good point.
There's a bit of legacy baggage to unpack to explain that one.
The *actual* "closed device" JailBreak in that package is limited to the FW range specified here.
But, in the package live a host of other JB-related tools that have much laxer FW requirements

.
Most notably the bridge, hotfix, emergency, and serial JB bootstrap scripts

.