That's a gross misrepresentation of what SMT is to the point where nothing you said is correct.

First of all. In SMT there is only one instruction decoder. SMT merely adds a second set of registers, which is why it is considered a "free lunch". The cost is small in comparison to the theoretical benefit (up to 2x performance).

Secondly. The effectiveness of SMT is workload dependent, which is a property of the software and not the hardware.

If you have a properly optimized workload that makes use of the execution units, e.g. a video game or simulation, the benefit is not that big or even negative, because you are already keeping the execution units busy and two threads end up sharing limited resources. Meanwhile if you have a web server written in python, then SMT is basically doubling your performance.

So, it is in fact the opposite. For SMT to be effective, the instruction decoder has to be faster than your execution units, because there are a lot of instructions that don't even touch them.