Good question and the origin of the myth actually started with a scientific paper.
The original study that started the claim that possums eat 5k ticks a season took several animal species from the wild, put tick larvae on them, counted how many they ate, and used those numbers to get an estimate of how many they could theoretically eat in the wild. The results of that paper were widely publicized and quickly spread as a reason to like possums.
However, newer studies, like this one, studied wild possum stomachs and found NO ticks in them at all. They also looked at the literature about possums and ticks and found that possums can carry ticks and that other studies of possum diets don't support them eating many at all. If they do eat ticks from grooming, the amount is a negligible part of their diet and maybe like a couple they eat from grooming themselves. They also found that the original study was highly flawed. The study didn't count how many tick larvae fell off the possums and assumed any that weren't on them were eaten. And that they based the estimates off of 100 tick larvae per species tested. The tick test was also done on possums in captivity, and animals in captivity act differently than those in the wild so they should not have just made the estimate and ran with it.
If you'd like an easier to read version of the newer study, this paper does a good job summarizing it.
Personally, I hate seeing the tick fact being spread around because of the idea that animals need to be useful to humans somehow to be worthy of life. I see stuff online frequently that people learn the debunked tick fact and use that as a reason to like possums. Even people saying that them supposedly eating ticks is reason enough to not outright kill them. But why do they need to be useful to us just to be appreciated? Possums are wonderful creatures and they don't need to eat ticks or whatever to be an important part of the ecosystem and admired for their unique characteristics.
