Land in this context is more of a mathematical concept: yes, you can make more land, but the surface of the oblate spheroid that we call "Earth" has a finite surface area, and it's that finite supply that gives land its population-proportional value.

Even if we're willing to stretch into a third dimension, that oblate spheroid has a finite volume, and the overwhelmingly vast majority of it is inaccessible to meaningful possession/occupation; you can only dig so far down or build so far up.

Two responses to this: Georgists classify “land” as both territory and all natural resources (really nature itself) so the “new land” the Netherlands makes is classified as an improvement to the pre-existing sea bed.

You might reasonably say this isn’t what most people mean by “land” to which the reply is, the amount of “new land” you can make in this manner is pretty limited and usually the reserve of states or state owned enterprises.

Comparatively, nothing stops you from tripling the number of domains overnight.

TIL about Georgists, thanks.

Regarding whether it's "pretty limited", yes and no, imo:

Yes, compared to, say, the size of a continent, the amount of new land is tiny.

However, new land is usually made in highly sought-after (high-density, high-value etc) areas [citation needed]. So it would make better sense to compare the surface of the new land to the surface of the adjacent area.

For example, the polders in the Netherlands significantly augment the country's surface. The artificial islands in Tokyo Bay significantly add to the real estate of the harbour. The artificial islands in Dubai and Bahrain significantly add to the coastline.

I think that this is enough for OP's analogy between domain parking and land speculation to reasonably stand.