You are right, it is an event and as such part of the framework and its rules. I think what they want to point out with the word informal is the difference to a formal status or project closure meeting in waterfall projects. "During the Sprint Review, the Scrum Team and stakeholders collaborate about what was done in the Sprint. Based on that and any changes to the Product Backlog during the Sprint, attendees collaborate on the next things that could be done to optimize value." So the attendees collaborate. This is very different to a Product Owner presenting what was done and stakeholders giving standing ovations, which is not the purpose of the review, because "the presentation of the Increment is intended to elicit feedback and foster collaboration".
> I think that once "Sprint Review" is official > event and be mentioned as its important, why it is "informal" meeting?
Formality would constrain behavior and could potentially reduce openness and transparency. Honest and frank discussion is necessary in order to maximize the value of the event. That's what is meant here by "informality".