Cosmetics are potentially harmful to humans, and that harm can be prevented by testing on animals first.
Maybe there is some risk of this tinnitus treatment causing harm (could be something like https://en.wikipedia.org/wiki/Havana_syndrome), and animal testing would avoid that. But it's not necessary to inflict extra suffering to show that. Maybe they had to show the combination of the treatment with tinnitus doesn't cause harm but I doubt that.
There's also an argument where a disease is rare and inflicting animals with it to allows for efficacy testing, but tinnitus isn't rare, plenty of human volunteers are available. (I mean, half the people in this thread have already "volunteered" to perform testing of possible treatments on themselves, myself included).
Lastly as others have pointed out there are many causes of tinnitus. The researchers only inflicted one kind of it (I'd assume it's the same kind that everyone hears after hearing loud noises, since these guinea pigs were presumably otherwise healthy), it's possible this treatment only works on that kind and is a waste of time for everyone else.