That's what the video says. So I'm asking, theoretically, if someone says "that wouldn't surprise me" about that, is it a form of lashon hara? Or motzi shem ra? Or something?
From the little check I have made indeed lashon hara in the halachic context is telling something that is true that is bad or can damage that person. Motzi shem ra is someone who tells lies about another person.
So, the issue here is what if a person is spreading rumors or theories that are unfounded but he believes they are likely or possible. I don't know halacha enough but it makes sense to me to avoid spreading such stories and if one just can't hold it in then at least he should say very clearly that it is pure speculation lacking evidence.