I don't understand why "that" is used before the plural noun. Sholudn't it be "those" instead?
Answer from a Native speaker
Ah, I can see why you'd think that! So here, this sentence is in relation to the previous one. So the full sentence would be: "Yet I know it's true that visions are seldom what they seem." So, "that" acts as a determiner that tells us exactly what she knows is true. Ex: I know it's true that she got the highest grade in the class. Ex: It's weird that no one can tell who Superman is because Clark Kent just puts on glasses as a disguise.