yeah this is not as surprising as it looks like

between pure fission design and thermonuke for a militarily relevant yields, say, 100-500 kt range, both designs are in principle working, but thermonuke is both compact and derives most of energy from cheap materials (natural to moderately enriched uranium and lithium deuteride). This is important if you remember that this thing has to fit in an ICBM

thermonukes have an extra advantage that they’re staged, that means dial-a-yield becomes possible - not all parts have to be used