There is no "safe" chemical treatment that will kill all invasives. Whatever was in the "dip," it was not the silver bullet.
Dydimo is actually the most fragile of the big 4 Aquatic invasives (Dydimo, NZ Mud Snails, Whirling Disease, and Zebra Mussels). Whirling Disease is the most difficult to kill with a myxospore stage that is resistant to chemicals, freezing, drying, aging, biodegradation, and digestion by gastric acid.
"The (Whirling Disease) myxospores can tolerate freezing at -20 centigrade for at least 3 months and are still viable after the passage through the guts of predators.... There have been reports from Europe of myxospores remaining viable in dry pond beds for 12 years(Bauer 1962)."
In the second paper that is referenced on WD states, "The contents of the myxospore are sealed by a protective shell making the myxospore highly resistant to stresses such as smoking (Wolf and Markiw 1982), aging, freezing, chemical exposure, and digestion by fish-eating birds and fish (Hoffman and Putz 1969; El-Matbouli and Hoffmann 1991). The myxospore can withstand temperatures from -20° C to 60° C (Hoffman and Putz 1971; Hoffman and Markiw 1977) and can resist biodegradation for years while retaining its infectivity (Halliday 1976). These resilient features of the myxospore make it likely that it will persist in an environment until it is ingested by the oligochaete host.
Whirling Disease is the "Terminator" of invasive species. It'll always be back.