I personally don't have much experience with slippery elm long term but it has similar properties to okra (mucilage) and it is used as a soothing agent (antiiflammatory) to gastrointestinal lining and helps moisten it. I think if combined with enzymes it helps coating the mucous buildup and the enzymes may help "digest" whatever is trapped in it.
I guess fighting mucous with more mucous (mucillage) can be helpful specially if combined with enzymes and/or bulking agent (fiber) to physically scrape it off.
Mike