WIY is essentially correct.
de Finetti’s point is that probability can be used to develop beliefs, but does not have any objective reality.
I brought the quote as a counter statement to yours- the fact that an event is a “scientific impossibility”, i.e. statistical probability near 0, does not in any way shape what has happened or what will happen.
If tomorrow morning the sun doesn’t rise (probability of near 0*), you would not be able to pretend it did not happen simply because it was too unlikely.
* I say near 0 because on the day when “Shemesh B’Givon Dom”, the sun in fact did not rise in this part of the world. Now you can’t discount the fact that it happened simply because it is impossible statistically. Because probability does not exist (except in your mind).