The jury is still out on exactly how intelligent other hominins were (and the extent of our involvement in their extinction). Regardless, the term human can apply to all of genus Homo and that's the sense that discussions of "human intelligence" typically use.
The burial sites of Homo Naledi suggest they had developed burial rituals and primitive art which would mean non-Sapiens hominids were likely smarter than previously believed
I like that theory although it is depressingly grim ... the top dog species will inherently see any alternative intelligence as a threat and eradicate it. Would definitely make one pause for thought about the wisdom of creating an AGI ...
I think it might be just competition. Human brains are expensive in terms of energy expenditure. So at certain population scales having less energy to expend might be comparative advantage.
And same really goes for other niches we do not even occupy. You need to get something out of those expensive to keep brains.
Since the invention of contraception, intelligence seems maladaptive, although we may have already reached technological escape velocity so it's not clear to me that it'll matter.
There are huge reserves of the population where contraception isn't a thing. There are so many people alive today, that evolving out intelligence is really hard to imagine. Perhaps in some kind of far future science fiction "robots spoonfeeding drooling humanoids" scenario.
Less extinction, and more evolution.