I think that's true to a pretty good degree, especially if they believe in the outcome as if it's a sure thing. Because nothing is guaranteed, though that goes both ways. (there's always good possibilities)
Optimism is actually healthy in general, but someone who puts their hopes in something unrealistic may get disappointed (though if they can learn to recover then it's fine.)
The thing is that faith is actually powerful, and some people may be meant to grow on that side of things, which can also help with support and upliftment in general. The problem comes when they try to talk/debate with people trying to be as realistic as possible. There's no way you'll win, so you might as well just say 'I believe she'll be in' and stop there. (and be optimistic in general, which feels good and also often helps others)
-though some people think they're realistic when they're actually pessimistic, which tends towards looking at the worst cases (only one side of the bigger coin)
An optimistic analytical approach can look at the data and see the good connections (hopefully remembering a bigger picture) and still have much better reasoning. Pessimism is good for strength-testing since it looks at the worst cases and little things they think could poke a hole in reasoning. (this can be annoying when still developping things, or invested)
Lots of things can be unhealthy when it's taken too far and unbalanced, and at most they only save themselves from a few things, and pessimism in too big doses comes at the expense of feeling badly about things liked in general. A little is fine in some ways though
But yeah, we can't be sure of anything, and that's ok, and we can always look to good things to come (& just support)
well that's my take anyway