There are important and significant differences between "Must Be True" and "Could Be True"
One way to think about it is this:
Another way to say "Must Be True" is "CANNOT Be False"
But another way to say "Could Be True" is "COULD Be False"
When you are saying something could be true, you are saying that you don't know whether it's true or false. In other words, you cannot rule out that it is true.
Hope this helps! Feel free to tag me if you have any follow up questions
