The notion of a species only really exists in the human world; it's a human concept. So the discovery of a new species is the discovery of a new classification need. It is new to science.

(At a first approximation, essentially all existing species are new on an evolutionary level — isn't it the case that something like 99.9% of all life on earth is newer than a million years old?)

It’s a new discovery for humans.

that's precisely their point

Not seeing the "problem." To humans, it's new. It's a new addition to the list of "species we know about." It's a new spider species. This is how language has been used for a long time.