 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 iswith 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.