If you want the actual opposite of your sentence, perhaps this would work:

Falser non-words will always be non-spoken.

Your example becomes:

"I am truly grateful", said Alfred. "Falser non-words will always be non-spoken.", snided Shelly.

Fits perfectly with snided, right? Shelly is one clever individual. :-)

If you want the actual opposite of your sentence, perhaps this would work:

Falser non-words will always be non-spoken.

Your example becomes:

"I am truly grateful", said Alfred. "Falser non-words will always be non-spoken.", snided Shelly.

Fits perfectly with snided, right? Shelly is one clever individual. :-)

Update: As per @Nossjimiki's comment, one might use unspoken in place of non-spoken, in which case the OP's example becomes:

"I am truly grateful", said Alfred. "Falser non-words will always be unspoken.", snided Shelly.

Works fine.

If you want the actual opposite of your sentence, perhaps this would work:

Falser non-words will always be non-spoken.

Your example becomes:

"I am truly grateful", said Alfred. "Falser non-words will always be non-spoken.", snided Shelly.

Fits perfectly with snided, right? :-)

If you want the actual opposite of your sentence, perhaps this would work:

Falser non-words will always be non-spoken.

Your example becomes:

"I am truly grateful", said Alfred. "Falser non-words will always be non-spoken.", snided Shelly.

Fits perfectly with snided, right? Shelly is one clever individual. :-)