All fungal diseases are easier to prevent than to treat. If your other snapdragons do not have the disease they should be treated prophylactically.
I would myself get rid of the diseased plants. Bag and trash. According to the article the fungus needs a living host.
I haven't grown snapdragons in years. They did better in drier weather. Humidity here is always 80% or better. Water by drip irrigation to avoid wetting the leaves.
The good thing about snapdragon rust was that it was pretty host specific.
This was not a perennial plant for me, it needed plant supports or staking which I really don't like to do. I have switched to other plants in my border garden that need less attention. I don't even stake my glads, even though I probably should and they still come up year after year (originally planted around 1992).
If you really love them, then try again when the weather is drier. If you are buying plants of any kind, check them out first. Buy a loupe, I found some for under $5 or just a decent magnifying glass. Isolate your plants when you get them home. If there are any problems most stores will take them back in the original pots with a receipt. I go to the nearest stores quite often, so I know what days they get plants delivered. The nurseries do a better job of feeding, watering and managing diseases than the stores.
Plants that are prone to diseases, especially fungal ones should be prophylactically treated with a fungicide if the weather conditions are conducive to fungal growth = warm humid days after rainy weather.
Neem has good antifungal properties and works best applied before there is a problem. Neem may be implicated in hive decline, so I have also used horticultural oil sprays before or within 3 days of rainy weather. The oil makes the plants more water repellent.
If it is too hot to use the oils then I spray with a sulfur based spray. Just pick one. Do not use a sulfur or oil based spray within two weeks of each other or the plants will burn.
https://ag.arizona.edu/plp/plpext/diseas ... nrust.html