There's not another explanation. Turbofan engines, opposite to turbojets, have a high bypass ratio (8:1 or so) which means almost 8 times as much air passing through combustion is bypass air. Turbofans generate most of their thrust with the 'cold air' rather than with the 'hot air', so it is adequate to use the bypass air for reverse thrust. On 'bucket' type reversers the whole thrust is diverted.
On the 737-200 (although I believe these are torbojets)
On the 717-200
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Photo © Marc Sauthier
Good looking aircraft the 717...