I'm afraid, you're missing Vaughn's point. He's talking extrinsic sources of image attack.
From the instant of its creation, a silver-based image faces attack from a variety of sources. Some are internal and essential to the materials photographic papers are designed and manufactured with. They come in the form of chemicals, inherent or added to the paper, the emulsion or the coating.
Other sources of attack are of external origin. Nevertheless, some are intrinsic to the photographic process and can be minimized but not completely avoided. Most processing chemicals fall into this category. In the very beginning of a printís life, and only for a few minutes, we need them to be present to complete their designated tasks. Beyond that point, we like to rid the print of them quickly and entirely. Fortunately, these sources of image deterioration are under our control, but no matter how attentive our work might be, unavoidable traces of them will remain in the print forever, and given the right environmental conditions, they will have an opportunity to attack the very image they helped to create.
The remaining extrinsic sources of image attack are hiding patiently in our environment, ready to start their destructive work as soon as the print is processed and dry. They can broadly be separated into reducing and oxidizing agents. Roughly until the introduction of the automobile, reducing agents were the most common sources of image deterioration. Then, oxidizing agents like aldehyde, peroxide and ozone took over. Their presence peaked in the Western World around 1990 and fortunately began to decline since.
In conclusion, Vaughn is correct, the mounting tissue will protect the print from extrinsic sources to some degree. Alternatively, people have used plastic barrier sheets behind the mounting board, but this creates other issues, such as trapped moisture. As Vaughn said, I wouldn't lose sleep over it, but it is a benefit of drymounting.