They're not alone in their view of it being a metaphorical fiery furnace. The problem is that they also reject the idea of it being a reference to hellfire/eternal damnation etc etc. It's not the only reference in Matthew to the whole thing either - Matthew 8:12 gives a similar sort of place.
Not a believer in the slightest, but I can't think of way to get Matthew 13:42 to fit JW doctrine without excessive use of derp. But then JW doctrine does this sort of thing a lot. It's doctrine by selective, context free, quotation.There's no internally consistent way in which they use canon scripture. Everything is literal until it's too inconvenient, and then those bits need interpretation back to what the writer really meant which miraculously always turns out to fit JW doctrine. Praise Jehovah. That approach is even set out as one of the things they did for the recent NWT revisions.