It's the Socionics way.
Uploading a video of you being emotional, then having someone typing you as an "F type" doesn't somehow magically bereft you of having any logical thoughts or logical abilities. This is why basing it on purely observations don't work, because which observations are we talking about?
A much more sensible way would be to say being emotional and logical are both within the realm of possibilities. Of course we don't know if Greta is actually capable of having logical thoughts, if all we see are 10 second clips of her being emotional. But we can also say that the human brain is capable of having any kind of rational, logical thoughts. But at the same time, they're also capable of having irrational, illogical thoughts.