The argument against p-zombies is that the reason for our talk of consciousness is literally our consciousness, and hence there is no reason for a being not otherwise deliberately programmed to reproduce talk about consciousness to do it if it weren't conscious.

A functional duplicate will talk the same way as whomever it is a duplicate of.

A faithful synaptic-level silicone WBE, if it independently starts talking about it at all, must be talking about it for the same reason as us (ie. consciousness),

A WBE of a specific person will respond to the same stimuli in the same way as that person. Logically, that will be for the reason that it is a duplicate, Physically, the "reason" or, ultimate cause, could be quite different, since the WBE is physically different.

since it hasn't been deliberately programmed to fake consciousness-talk.

It has been programmed to be a functional duplicate of a specific individual.,

Or, something extremely unlikely has happened.

Something unlikely to happen naturally has happened. A WBE is an artificial construct which is exactly the same as an person in some ways,a nd radically different in others.

Note that supposing that how the synapses are implemented could matter for consciousness, even while the macro-scale behaviour of the brain is identical, is equivalent to supposing that consciousness doesn't actually play any role in our consciousness-talk,

Actually it isn't, for reasons that are widely misunderstood: kidney dyalisis machines don't need nephrons, but that doens't mean nephrons are causally idle in kidneys.