The title of the research article
https://www.cell.com/current-biology/fulltext/S0960-9822(25)...
is more informative: "Cambrian origin of the arachnid brain".
This has nothing to do with spiders specifically, but only with a larger group of arachnids, which includes at least scorpions and spiders, together with the less widely known pseudoscorpions and pedipalps.
Among the chelicerates, besides groups that are completely extinct, fossil sea spiders and horseshoe crabs are known from Ordovician, terrestrial relatives of the scorpions and spiders appear later, in Silurian, and even later, in Devonian, there are also fossil mites and harvestmen.
This article is about an earlier fossil, from Cambrian, which appears to be closely related to the group that later will become terrestrial, then diversify into scorpions, spiders and their relatives.
The arachnids are divided into 2 groups, one containing scorpions and spiders and their relatives, and another containing mites, harvestmen, ticks and their relatives.
It is not known yet whether these 2 groups have split after becoming terrestrial, or they have split while still being marine and they have transitioned to life on dry land independently. The scorpion-spider group originally had lungs, while for the other group it is not known whether they had originally lungs, then all had lost them, or they never had lungs. Most smaller arachnids of the first group have also lost their lungs, because at smaller sizes trachea are more efficient than lungs.
From the data presented in the article it cannot be inferred with certainty whether this fossil was closely related to the ancestor of all arachnids, or only with the ancestor of the group including scorpions and spiders, though the latter variant is slightly favored.
The reason for the uncertainty is that fossils do not preserve enough characters to determine accurately the phylogenetic relationships.
Because they have used only the characters testable in the fossil, the phylogenetic tree drawn in the article has an error that is certain, a solifuge (camel spider, Eremobates in this case) is shown as more closely related to scorpions and pedipalps than the spiders, which is wrong beyond any reasonable doubt (scorpions, spiders and their relatives have a duplication of the genome that does not exist in the other group of arachnids, so there is no doubt that the groups are separated in phylogeny).
Having one error in the extant species phylogeny, which mixes the 2 groups of arachnids, makes uncertain the relationship of the fossil with them.
In any case, this pushes back the history of the arachnids with many tens of millions of years and further studies may help to understand the history of the arachnid evolution.
Currently, the phylogeny of the arachnids and the history of how they have adapted to land life is one of the most mysterious parts of zoology. The phylogeny of the group containing scorpions and spiders is well understood and also how they did become terrestrial. However the relationships within the other group of arachnids and the relationship between the 2 groups, and also with the horseshoe crabs, and the history of their transition to terrestrial life, are still completely uncertain. Even for the horseshoe crabs, it is not certain whether the currently surviving species have always been marine, or they had been terrestrial at some point in time, then they have reverted to a marine lifestyle.
