Guessing is just saying whatever random idea comes into your head. It's gut instinct. A hypothesis is formed when one thinks about what one knows and then decides that X is the likely outcome. It's not guessing.
That's just saying the same thing twice, the second time with pomposity.
The true distinguishing feature of a worthwhile guess/hypothesis is that it comes with its own means of evaluating its validity. That is, any guess must contain an objective criterion, a hinge point for the teeter-totter between true and false. And this criterion must hinge on data that is not
yet available, so that one can then objectively seek the bit of data and see what it tells about the guess.
Say, Spock could tell Kirk that he believes the thing that ate Gamma Seven and the Intrepid
could be a malevolent lifeform or intelligence that seeks victims, or an indifferent natural phenomenon that stumbled upon Gamma Seven after which the starship intercepted her to her own peril. He would then specify that the data he lacks is on whether the phenomenon homes in on victims, and suggest as the litmus test that Kirk stand off and wait whether the thing moves to attack. If not, the thing might still be maneuverable but wary, so Kirk should provoke it with, say, a phaser attack.
This would bring Spock's scientific mumbo-jumbo to Kirk's area of competence and allow him to choose between options, rather than just chauffeur the starship deeper into the danger zone in an aimless pursuit of more data.
Forming a hypothesis that hinges on already known data is just masturbation; it involves subjectively interpreting that data. And "thinking about what one knows" does not make it any less subjective or any more convincing. It just takes one deeper down the path of self-delusion.
Spock is being fundamentally unscientific if he thinks there is insufficient data to form a hypothesis, i.e. to make a guess. A hypothesis is formed when data is lacking. If one believes one is in possession of a full dataset, then the time of hypotheses is well past, and there's no science remaining to be done; the truth is already known...