This seems like a bug in the breaking change detector - why would changing the type to a supertype (e.g. adding another type via union types) be considered breaking?
This is universally considered a non-breaking change AFAIK, and I cannot think of any scenario where this would break in someone's code.
It could be that this library flags ANY change to a method as breaking? But I don't think so, as that would be pretty excessive and I feel like I would have noticed that by now...