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.

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.

Stellar fusion could also be a factor in a sufficiently large compost heap.

Of course it's misleading, thats the point GP was trying to make. Averages belie truth in large, diverse systems with extremes