I interpreted the quotes as implying that the children hadn’t developed a full human language, but rather the rudimentary beginnings of one. Independently of whether it’s signed or spoken.
(You are indeed correct that sign languages are full-blown languages, not just re-encodings of the local spoken language as many people think.)
But that's not what (scanning back through the article) the author used quotation marks for. They mostly talk about children forming a sort of pigeon in a laboratory setup.
(You are indeed correct that sign languages are full-blown languages, not just re-encodings of the local spoken language as many people think.)