It only seems to document that there's group scheduling for SMT cores. But that doesn't prevent issues due to switching between customers on the same physical core, no?

"It is possible, however, for two burstable performance EC2 instances to run sequentially (not simultaneously) on the same core. It is also possible for physical memory pages to be reused, remapped, and swapped in and out as virtual memory pages. However, even burstable instances never share the same core at the same time, and virtual memory pages are never shared across instances. "

I only started reading the paper - so I very well might be wrong here - but it doesn't look to me like you need victim/attacker to be scheduled simultaneously on two SMT threads, but that a single core sequentially executing victim / attacker code would be vulnerable. It's possible that the cross-customer "context switch" is larger than the the vulnerable window, but I'd not want to bet on it.