The train of thought that leads to that belief is usually along the lines of: We're the only sample we have. It's more likely than not that what our planet and ecosystem has produced is not an outlier but the norm.
That being said, of course I strongly believe those to be fake and also assume that there is a huge amount of variance in what intelligent life with potential to develop spacefaring technology could look like. Therefore we're probably not an outlier, but the possibilities within non-outliers are still so vast that our first contact would likely look a lot different.
I mean even on our own planet, there’s such a wide variety of life and most of it looks widely different from humans aside from fellow primates. Why should they look anything like people, and not like cephalopods, or insectoids, or more likely something vastly different in design. They would have an entirely different evolutionary lineage - and who’s to say they’d even have a genetic system like our own? Like you said, we’re the only sample we have. You can’t make assumptions about a galactic or universal population based on one planetary sample size
Of course, this could be entirely wrong, and maybe extraterrestrial life is more similar to us than we’d expect - but even then, there’s no reason to expect any life forms we encounter to look like us any more than we should expect any other species to. Sure, a few may, and there may be similarities, but to assume they’re likely to be at all humanoid just feels like heavy anthropocentric bias
Convergent evolution is a thing. Aliens that live in similar natural habitats and that fill similar ecological niches as humans will likely evolve along similar lines due to facing similar pressures. With that said, this is a very big assumption and all that it really tells us is that it is not impossible for "humanoid" alien life to exist.
Also, whatever is out there, we'll never know for certain. Distances are just too large. Space is so incredibly, unfathomably big that it's almost certain that there's other intelligent life out there. However, space is also so incredibly, unfathomably big that it's almost certain that we'll never meet it.
It totally makes sense if they're a time traveling, evolved species from our own just trying to learn about its past. Or maybe just site seeing. Either way, their trip didn't seem to end well.