Astra says this claim is misleading. Sun's average energy output per cubic meter is three orders of magnitude smaller than that of a compost heap. The fusion core is comparable though.
Of course it's misleading, thats the point GP was trying to make. Averages belie truth in large, diverse systems with extremes
The energy density of specific compost heaps varies by several orders of magnitude it’s not a single number. Volume, moisture content, internal temperature, external temperature, materials being composted, etc all play a significant role.