Sunset may think that she could have made friends with people that share similar interests to her, but this could still prevent her from wielding the Elements, because even if she thinks she could have made friends it would most likely be a relationship of colleagues and she can still try to keep a distance from them and may not become that close to them.
Remember she is an anti-social and somewhat arrogant person.
For all Celestia could have known Sunset could have resented people who who were as smart, smarter, or tried to become as smart as her and needed some humility.
And before anyone starts badmouthing me that this humility takes the form of making someone feel down, let me remind you that we are talking about a highly arrogant and anti social person that wants to become a god, you don't give someone like that that much power unless they are humble, or at least have better control over themselves and their impulses.
Sunset has done things that put her so called genius into question.
She is a brilliant scholar and mage, but in everything else...ehh
Celestia isn't trying to make her feel down about her accomplishments and trying to put her down.
She just discovered that her student, who she probably thought was dead for over a decade, was alive and well and had made friends, something that she wanted for her.
She is like a parent who just discovered their child was alive and doing well, OR COURSE she isn't going to think in a totally rational manner and may fixate on certain things, because she is extremely RELIVED,
And her being suspicious of the renegade is logical, because there is a complete unknown in her home that may have done something to her former student.
If Luna, or the Mane 6, showed her their memories of him she may still be suspicious because he could have manipulated them.
Even if Sunset gets her 'apology' from Celestia like she wants it, it will be a hollow one, or at least even if it is genuine it won't be like the one she wishes.
Zoat is a brilliant writer and he has created scenarios that make his characters think about what they want and what getting it will feel like, i really hope that he doesn't just write one where Celestia just apologizes to Sunset.
She can apologize, but i get the feeling the taste of victory will be bitter.
