The prose is unclear, but I expect by arrival they mean success/dominance, in the sense given by the phrase "truly arrived." See the definitions given at https://idioms.thefreedictionary.com/have+arrived. Seems like poor phrasing rather than straight up contradiction.
Moreover, while most animals preserved in fossils after the start of Cambrian (540 million years ago) can be clearly identified as relatives of known animals, most of the fossils dated before the start of Cambrian and presumed to be of animals cannot be determined with any certainty as relatives of known animals. There are various hypotheses about the nature of the Ediacaran animals, but none of them is reasonably certain.
The exception is that some Precambrian fossils come without doubt from some kind of sponges.
Unlike multicellular animals, which have appeared much later, multicellular red algae and green algae were already widespread one billion years ago.
