It was my understanding Amazon does in fact do this for most instance types, it's why the amount of RAM and other resources scales with CPU core count.
There are exceptions though in the very small "burstable" etc instances, like the micro etc.
EDIT: Seems I am wrong.
"Each vCPU is a thread of a CPU core, except for T2 instances and instances powered by AWS Graviton2 processors."
The exception for Graviton here is because every ARM vCPU is a full core, whereas an Intel/AMD vCPU is one hyperthread (i.e. half a core). They're not saying Graviton2 cores are shared. For Intel/AMD, you always get allocated both hyperthreads on a dedicated core; that's why hyperthreaded families start at 2 vCPUs. Only the "T" families feature shared cores.
There are exceptions though in the very small "burstable" etc instances, like the micro etc.
EDIT: Seems I am wrong.
"Each vCPU is a thread of a CPU core, except for T2 instances and instances powered by AWS Graviton2 processors."
From: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance...