Creating mesh item

It's in the Javascript Reference, but even if it wasn't there, not everything you can do manually is available in the Scripting API. Other functions are also not available, ie access to Appearance panels among many other functions.

"MeshItem
A gradient mesh art item. You cannot create mesh items from a script. However, you can copy an existing
mesh item with the duplicate method, then use the one of the move methods to place the copy at the
proper location."