= Solution
The inequality $m_H<2m_W$ forbids two on-shell W <bosons> in a two-body final state. It does not forbid the four-fermion states reached through a virtual W propagator. The physical channel is
$$
\boxed{H\longrightarrow WW^*\longrightarrow4\ \text{fermions},}
$$
with at least one W off shell. The virtual particle need not satisfy $p^2=m_W^2$; the final stable particles still obey total four-momentum conservation. The W propagator and reduced <phase space> suppress the rate relative to an above-threshold two-on-shell-W decay, but do not make it zero. The label $H\to WW$ commonly includes this off-shell contribution to <Higgs decay to two W bosons>.
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