Lol everyone has the right to make predictions but that doesn't mean your prediction is right or wrong, even the person so confident might be getting it wrong so they are just mere predictions that might not happen.
There is difference in squad strength and it's obvious that Chelsea are stronger than Betis but that doesn't rule out the possibility of Betis beating Chelsea. This is a final and all teams will definitely come prepared. Chelsea have had the mindset of qualifying for the Champions league or Europa league even when they didn't have hope in making top 4 they were determined to win the Conference league and get a spot in the Europa league at least but to their surprise they made top 4 and now in the champions league now they will want to win the title to have a more successful season.
Chelsea could lose as you mentioned but I would be surprised if they do.
Prediction is something that might happen like you said and nobody can say he or she knows what will happen next, we only speculate or predict.
I don't think Betis has a weak squad, they are better than Chelsea. To me Chelsea have a better squad no lies but the manager don't know how to use the players to be one of the best and when you see Betis play they don't have any body to say without him the squad can't perform well but at Chelsea when you take out Palmer the whole team becomes confused, they can't operate well and that's something that have been affecting Chelsea.
Chelsea was lucky to book a place in the CL next season and if they play the way they did against Forest I don't see them enjoying themselves in the finals, it will be a disaster. This very game between Real Betis vs Chelsea might end in a draw which at extra time the trophy might go to Spain.