Niagara doesn't do speculative execution, and their SMT model is wildly different from intel's - specifically, they are a variation of barrel cpu where you duplicate minimal amount of resources to hold state then execute X instructions per thread in round-Robin fashion. Similar setup is used on POWER8 and newer (which allows you to dynamically change the amount of threads available).
The CPUs still include speculative execution (starting with T3 Oracle introduced speculative and out-of-order execution in the S3 pipeline) and Oracle had to release patches to mitigate Spectre v1 and v2 vulnerabilities; see Oracle Support Document 2349278.1
