There is not much real evidence for "devs wrongly assume" and as someone writing numerical code (clusters, NUMA, SIMD, etc.) I think C is still the ideal tool for this.
Assuming you mean the standard does not provide features for numa and simd? It doesn't necessarily have to. I think it is not surprising that you seem always bewildered that people still use C (as per your comments), as it seems you fundamentally do understand neither standardization nor systems programming.