50% only occurs when ALL cores AND threads are fully engaged by optimized software. It is an absolute best case.

But the reason that many gamers underestimate what HT/SMT can do is because they count non-utilized threads.

For example, let's compare the i5-8400 (6c/6t) to the i7-7700K (4c/86). At the same clocks, and if a logical core = half of a physical core, they should be at equal performance, right?

The problem with that theory is that it would only apply if all cores and threads were fully and optimally utilized. If a game uses 6 concurred threads, it becomes 6c/6t (8400) vs. 4c/6t (7700K). And when you look at it like that, the 7700K would fall being (again, in this theoretical experiment where we've equalized clock speeds and fully loaded each core/thread in question). The 2 additional threads of the 7700K would only become relevant if the software were to utilize them.

So yes, HT/SMT can reach ~50% of the performance of a physical core, but it's insanely rare in real-world scenarios.