It is the (constitutional) capital according to Wikipedia, whereas La Paz is the seat of government / administrative center. Do you have a better source for your claim?
Sucre / La Paz is the same as Amsterdam / The Hague for the Netherlands (capital / seat of government). Would you analogously argue that Amsterdam is NOT the capital of the Netherlands?
It would be nice to mention that both capitals have to be named for countries which have more than one. I didn't think of Sucre because La Paz was already entered, but theoretically, Togo and South Africa have to be considered, too.
I think you're confusing Togo with Benin since Lome is Togo's only administrative Capital which is Lome, while Benin does have more than is one Capital which are Cotonou and Porto-Novo