I've never used either of the packages that you've mentioned, but I did some digging and was able to find another user who was struggling with the same issue. Apparently the documentation you're looking at is referencing the Network type, so you'll need to instantiate a Network in order to use the getSeed() method

The answerer on the actual question page explained it more clearly, check it out: vis.js - how to run the getSeed() method

Hopefully this helps a bit!

If you look at the source at neovis.d.ts, you will see this line:

/**
 * The vis network object
 * @link https://visjs.github.io/vis-network/docs/network/#methods
 */
get network(): VisNetwork.Network | undefined;

And in your Neovis script you should have this:

viz = new NeoVis.default(config)

This is equivalent with var network = new vis.Network(container, data, options) in Vis.js' tutorial. So all you need is to use viz.network.getSeed() in the console.