I think your confusing objective with universal. They aren’t the same thing. You can have an objective truth that isn’t universal. Let’s use Euclidean and non-Euclidian geometry as an example, both have very objective truths, but they aren’t the same or interchangeable.

The problem with saying slavery is universally immoral is that you label most everyone that existed prior to the modern era as immoral, because slavery was foundational across the globe in multiple cultures. Morality isn’t useful if you can actually compare or contrast because even involved was immoral.