I do not agree. Different degrees of meaning exist. If something is "almost tangible," that means it is not quite. That is a different and more narrow expression than "not tangible," the only expression you are recognizing.
I am not just recognizing silence or no silence, but think that a more presice description would serve their intent better, as I mentioned above with "the low chirps and other sounds uncharacteristic of their normal chatter." The reality was that the chimps were being uncharacteristically quiet, not "almost tangibly silent," which, again sounds fucking stupid.