Worms World Party is the last game of its generation to use the same engine. The reason why the updates are for Armageddon and not World Party is mostly arbitrary - it was the game that the original maintainer had and was playing. A bit more info here:

https://worms2d.info/WA_vs_WWP

There is also Worms W.M.D., which we understand is based on Armageddon's source code, but that came out much later, and is still very different in terms of gameplay and style.