Amazon is pretty clear that Gravatron is an actual core, while the others are hyper-threads[1]. It makes sense that at higher vcpu counts you're likely to run into contention with neighbors, or even yourself.
I believe AWS always gives you both hyperthreads on a physical core, you never share a core with another customer. That's why hyperthreaded instance types always have an even number of vCPUs rather than starting at 1 vCPU.
Source: trust me bro, I can't find the relevant AWS doc.
One of the major differences between AWS Graviton2 instance types and other instance types is their vCPU to physical processor core mapping. Every vCPU on a Graviton2 processor is a physical core. This means there is no Simultaneous Multi-Threading (SMT) and more isolation between vCPUs. By contrast, every vCPU on a 5th generation instance type with Intel processor (such as M5, C5, and R5) is a hyper-thread. This means vCPUs share resources and there is less isolation than in the case of Graviton2.
What I mean is that you're always sharing resources with yourself, not another customer. They're assigning you both hyperthreads on a single core when you get an instance with "2" vCPUs; it's not one hyperthread on one core and the other hyperthread on another core, with some other customer taking the other hyperthread on each core.
[1] https://docs.aws.amazon.com/whitepapers/latest/aws-graviton2...