The title of the article is incorrect, the worms live in the crust, not "beneath the planetary crust" (in the magma).
The Economist magazine is not what it used to be, sadly.
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You have severely misunderstood the article.
It is not saying that there are tubeworms (and other animals) around deep sea vents. This, as you say, is well known.
It is saying that they also live underneath the sea bed in cracks in the rocks of the crust. This environment is even more extreme and brutal than in the open oceanic water. It is hotter, much lower in oxygen, and the chemical concentrations are even higher.
The organisms living within the rocks are even more extremophile than the ones in the oceanic water.
The greater point you are missing is that there is a whole other type of ecosystem that is less well known (and I think more recently discovered) than even vent colonies. The environment of organisms living in cracks and crevices of the rocks of the crust, far beneath the surface of the planet.
We've only drilled to ~12km down. The crust is up to 70km thick, comparable to the thickness of the atmosphere.
https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole
The point here is that much of that very large volume is inhabited, by organisms that have been down there for a very long time. Not just millions but billions of years.
Billions-of-year-old rocks contain living organisms:
https://www.discoverwildlife.com/environment/living-microbes...
This is a vast and almost unknown ecological niche, the deep biosphere:
https://en.wikipedia.org/wiki/Deep_biosphere
And now, as the very top part of it at the bottom of the ocean is explored, it has metazoans in it too. Multicellular life, animals, as well as protozoan.