I encourage you to read some of these nice sources you have selected. Allow me to quote:

> Such signing may be considered complex enough to be called a form of language if the inventory of signs is large, the signs are relatively arbitrary, and the animals seem to produce them with a degree of volition (as opposed to relatively automatic conditioned behaviors or unconditioned instincts, usually including facial expressions). In experimental tests, animal communication may also be evidenced through the use of lexigrams (as used by chimpanzees and bonobos).

and

> Seyfarth, Cheney and Marler reported that vervet monkeys (now called Chlorocebus pygerythrus) responded differently to different types of alarm calls2 (although some of the calls overlap acoustically3 and this view is currently debated4). More recently, west African green monkeys (Chlorocebus sabaeus) rapidly learned the novel referent of an alarm call that was given in response to a drone5. Referential signaling is not limited to primates.

You'll notice the parent did share sources! They presented a bundle of them, of actual cats using actual signifiers to refer correctly to signified objects.