There's neither surprise nor accident in Factorio's Turing completeness. The developers were aware of and fans of the existence of proofs of games on that list (Transport Tycoon is a particular example that they intentionally modeled train mechanics from)
>>Gauntl+(OP)
Obviously the circuit system is Turing complete on purpose, but I believe the belts and splitters may be Turing complete on their own, which would definitely be accidental and surprising to most.