Why did Satoshi design Bitcoin to be mineable only on specialized hardware, if the goal was decentralization? GPU-mineable cryptos are arguably more decentralized.

Did he simply miss this design aspect or does this sound fishy to you?
Bitcoin was not designed to only be mineable with specialized hardware. When Bitcoin was created in 2009, ASIC miners did not exist, SHA256d ASICs did not exist. Even GPU mining software did not exist because mining was a completely new thing. Bitcoin's difficulty was low enough for Bitcoin to be CPU mined on a laptop.

However over time, as more and more people began using and mining Bitcoin, the difficulty increased. Eventually someone figured out how to do GPU mining and wrote software for it, so Bitcoin moved on to GPU mining. Eventually someone figured out how to mine Bitcoin using FPGAs (Field Programmable Gate Arrays) so Bitcoin moved onto being mined with FPGAs. Eventually someone figured out how to build ASICs (Application Specific Integrated Circuits) for mining Bitcoin, so Bitcoin moved onto ASIC mining.

Satoshi did not intend for Bitcoin to be mined with ASICs (or even GPUs or FPGAs); it was meant to be mined on CPUs, but over time, as technology advanced, people figured out better ways to mine.