Three points about the tunnel.
Firstly, knowing
when it was built says npothing about
who had it built.
Secondly, as someone interested in folklore, I know that just about any prominent, but otherwise unknown artifact, in an area has a "just so" story about who created it and why. If the local population knew of the tunnel - and I believe it has been open and in use since it was built - when the books of the Bible were being written, and knew its approximate age - not impossible even from folk memory, and early written sources may have mentioned it indirectly, it is not improbable that it was simply attributed to whichever king in the Bible best fitted the known or assumed age.
Thirdly, I am extremely suspicious of the explanation - to provide water for a siege. Simply, it would take too long to build to have been any use in time even digging from both ends, the time to build would have been measured in years, not days. While almost certainly built to ensure security of water supply in a time of war, the concept of it being built because of a
particular threat is simply incredible. Even in the 19th Century, with the use of gundowder and steel, not iron, tools, tunnels took years to construct.
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