Ok, so… @HalukOzkul made me realize that when Epic is saying “mirror”, they do not intend to say “flipped”, but rather “bookmatched”… A simple example of that would be something like this:
which is “mirrored” inside the texture itself, meaning the mirroring is relative to the center of the image. And if I were to “un-mirror” this image, then yes, TexCoord’s behavior now makes sense… pretty sad their documentation is not very explicit and they give zero examples of the additional node features 
That being said, I managed to actually “flip” the texture the way I initially intended to, by messing with the Tiling value and setting it to -1.0 instead of the regular 1.0, this effectively turning my image the other way around:
So… case solved! 
Again, I just wish Epic would be more explicit with their intent inside the documentation, since this makes their extra features partially useless if we expect them to work in one way, yet they do completely different things, due to the developers having something else in mind, yet never bothering to explain it to anyone, anywhere…