Why Belgian neutrality was more than a treaty slogan

Does Britain's concern over Belgium explain the propaganda about a scrap of paper?

Yes. The treaty guaranteed Belgian neutrality, which mattered to Britain because hostile control of the coast threatened its security.

The phrase "scrap of paper" refers here to dismissing the treaty guarantee of Belgian neutrality. For Britain, that commitment also concerned the strategic danger posed by a hostile power controlling the Belgian coast. A coastline that could support an invasion made the treaty relevant to British security. The dispute therefore connected treaty obligations with a concrete geographical concern.