I assume they mean during the big conflicts because armies would be levied and trained specifically for conflicts instead of having standing armies. It's not absurd to think that during the napoleanic wars, the prussian expansion wars, and german unification wars that countries would be spending that much of their GDP. In peacetime though, i highly doubt it. Even during the naval revolution from 1860(ish) on through WW1 i doubt it reached 30% GDP, even the Royal Navy.