When you smoke at 180 degrees, you are smoke-cooking. In that neighborhood, the fat starts to soften and render and the protein of the meat changes significantly from the heat. All of that assumes, like the other fellow wrote, that the internal temperature of the meat gets high enough in a reasonably short period of time. The extent to which cooking occurs is dependent on the final temperature.
I really do not know why you would need six hours to get your meat to the required 160 to 165 degrees and that is one of the implications of what Squeezy wrote. Better you should place a meat thermometer into one of your sausages and measure just how long it takes to get there. At the required temperature, enough meat protein has been denatured by the heat to kill Trichinella spiralis. Once the trichinosis is no longer an issue, the amount of doneness is up to your preference. At those temperatures, many other bacteria have been killed, too, but not all. At 165 degrees, for example, Eschericia coli, a common fecal contaminant and disease producer will have been killed. E. coli is ubiquitous and will be found in any preparation that you do.
Bacteria grow and thrive between 40 degrees and 140 degrees. Some, such as botulism can survive 212 degrees plus. That is why nitrates and nitrites are added. Regardless of temperature, they suppress the Clostridium botulinum bacteria--they do not kill it. If there is no bacterial growth, there is no toxin produced and, hence, no sickness. There is some evidence that the nitrite makes the botulism easier to kill by heat, too.
Salt, while adding taste to meat and helping to develop other flavors, inhibits the growth of most bacteria at higher concentrations. That is the basis for adding salt to preserve meat. Salt is part of the curing mixture, although technically it does not cure the meat--the nitrate cures the meat. The salt, when distributed into the meat, sequesters moisture and prevents is use by the bacteria; the meat looks like a desert to the bacteria and they cannot grow.
Many tangy meats were so because the meat naturally contains Lactobacillus spp. bacteria. During the former long curing times, the Lactobacillus would produce Lactic Acid while the other bacteria were reducing the nitrate to nitrite. The added Lactic Acid produced the tangy flavor which people grew to like. The Lactic Acid also lowered the pH and made the meat more acid. Many bacteria find this acidic environment a poor place to live and multiply, so the original purpose of the tangy flavor was preservation, too. You can add one of the commercial flavoring acids to your product, if you like the flavor or if the recipe actually needs it to be correct for that type of sausage; the added acid may act as a preservative as well.
There are some other acid salts that are occasionally added: Sodium Ascorbate, the sodium salt of Ascorbic Acid, which is Vitamin C and Sodium Erythorbate the salt of Erythorbic Acid, the stereoisomer of Vitamin C. both act as anti-oxidants and help keep the fat in the meat from turning rancid, among other things.
Hope that helps.