In my opinion, it's a rumor that will never come true, so just ignore it.
Ronaldo will not return to Manchester United just to prove that Ten Hag is a coach who has made mistakes in his career. He didn't have a great season with Ten Hag and that can't be denied, but Ronaldo is a great player. In the SPL, Ronaldo shone and he seemed happy to be there instead of returning to Europe.
Most likely a rumor yes. I don't think Manchester United is an option for Ronaldo since he wouldn't return to a club that doesn't play in the Champions League. The guy is so ambitious that if for some reason he would miraculously return to Europe, he would only do it if he can prove that at the age of 40 he can still score goals in the most important football competition on the club level in the world. He wouldn't want to play for a sinking ship like Manchester United, whether there is some history that binds him to the club or not. He would rather go to a top 10 club and be the first striker in the team and then try to get that team as far as possible in the Champions League competition. But no, he is earning too much money and that will keep him either in the SPL or end his career altogether. But since he wants to keep playing for the national team, I think we will see him play in the SPL until at least 2026.
No, it not about Manchester United not playing in the UCL but because for now Ronaldo goals is just to have fun at the end of his career while still getting paid big and he will not return to Manchester United if Al Nassr does not extend the player contract.
I sure some other Saudi team will be interested in having Ronaldo because his performance in the SPL is really very satisfying, but I that there is doubt that Al Nassr will let Ronaldo go just like that unless it is Ronaldo own wish to leave Al Nassr.