You're not confused--AWS either gets custom chips without it, or they disable the SMT. I'm not sure which. Here's where AWS talks about it: https://aws.amazon.com/ec2/instance-types/m7a/
> One of the major differences between M7a instances and the previous generations of instances, such as M6a instances, is their vCPU to physical processor core mapping. Every vCPU on a M7a instance is a physical CPU core. This means there is no Simultaneous Multi-Threading (SMT). By contrast, every vCPU on prior generations such as M6a instances is a thread of a CPU core.
My wild guess is they're disabling it. For Intel instance families they loudly praise their custom Intel processors, but this page does not contain the word "custom" anywhere.
To compare to Heroku's standard dynos (which are shared hosting) you want the t3a family which is also shared, and much cheaper.