What they have actually shown - that cuttlefish react to another cuttlefish waving its tentacles - is clearly not showing that tentacle waving "serves as a communication system between cuttlefish.
No that is precicsely communication, but nothing indicazes that is "talk". Communication is the "lower" phenomenon ghen talk/speech. Involuntary body processes communicate something in us sapiensapiens and we are sure tjat animals communicate. But do they talk... if we say "talk" ("sprechen") that entails the whole of what natural languages do and is quite different.
I think you're confused about what the communication is.
The communication isn't the waving in "reaction" (it's not clear it's simply a reaction, but let's assume it is) to the original wave, but the original wave itself.
And the fact that it's also triggered by videos indicates it's not just a mechanical reaction (like some of the research about how plants "communicate" is which are essentially mechanical responses to stimuli).
However, this doesn't necessarily mean that the communication is meaningful. It just shows that a means of communicating exists.
Well data needs to be conveyed, as in there has to actually be a message. If one person yawns and that causes you to yawn, that is not evidence of communication - the first person was almost certainly not trying to send any information and you received none. Mimicking another's arm movement could be just as meaningless.
What they have actually shown - that cuttlefish react to another cuttlefish waving its tentacles - is clearly not showing that tentacle waving "serves as a communication system between cuttlefish.