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The boxy shape would likely still be less aero in isolation, but certain parts that in its slipstream might be so un-aerodynamic that the see less drag through more turbulent air than through clean air, and the boxy one provides more turbulence. Or it's the reverse, total drag benefits from the boxy rear and the rounded one was round on the rear as well (or not round at all, just some knotty wrap that approximates "round" very roughly, contrasting with a clean box)

The round-bottom shape that sits under clip-on aero bars is very good.

https://cyclite.cc/en/products/handle-bar-aero-bag-01

https://torstenfrank.wordpress.com/2021/12/07/aerodynamik-vo...

Yeah, the round shape is actually not very good. But it's the traditional one, from bicycle frames being welded steel tubes. The circle is still as wide as a square would be, and additionally it creates lots of turbulence. An aero frame is much more tear-shaped.