I didn't miss anything in the wordplay*, it was obvious. (As are the initials, an extra pun).
I put quotemarks around "flamethrower" because that's what it was originally sold as before obvious and predictable legal issues with real flamethrowers and the fact it was obviously mimicing the prop in Spaceballs.
My point is: neither weed burners nor actual flamethrowers have anything to do with digging tunnels nor any adjacent aspect of civil engineering.
But they sold a blowtorch aka not a flamethrower. The difference being a flamethrower actually "throws flames" like 10+ feet.
[1]: https://en.wikipedia.org/wiki/Bore