I think you wonderfully OP, especially having been caught off guard. You can pick up preferred language along the way.
I think if the 'things can change' thing is belittling depends on how it's done. When it's only done about sexuality (or careers, those two seem to be the big ones), yeah, that often comes off as patronizing and can be very disheartening & break relationships between parent and child.
With my kids, I discuss adolescence as a time of figuring things - looking around and asking 'is this me?'. Their father and I openly discuss thoughts we had at a young age and how many things changed and many things stayed the same, but I couldn't have told you which was which then, everything felt strong.
Also, I discuss sexuality as a trait, not a full identity or a shorthand for particular values or personality types as portrayed a lot in media and I'm very big on keeping personal information private especially in places where they are not free to leave like school. I emphasise that no matter the popular message, they are not less themselves or less 'proud' if they don't out themselves in every setting. It may cool in other places, but in my DD1's school, she's already dealt with a lot of vileness from people just suspecting she's a lesbian.
At 10, you are or should be an innocent child.
A 10 year old's feelings are innocent, even when they describe them as crushes or liking someone. Same with 5 year olds. It's adults who turn them into something they're not by applying adult emotions and motivations onto children who are stuck using the language given to them.
From what I remember when I was that age, we all thought they were repulsive creatures.
I thought I was repulsive at 10, but I never thought boys were repulsive - I gave one a pile of valentines at 6 and more than a few of my peers that I would now say I was in awe of were guys (back then I would have called it a crush, but my emotional language was limited to thinking an intense admiring emotion involving wanting to be around someone must be a crush).