Well unless they can somehow force your preferred supervisor to take you on for your preferred project, they're guaranteeing a place in honours but not your ideal one
I don't think your concern is a big concern in choosing to do honours. Research is very different to coursework. UGs don't generally have a preferred supervisor/project, because typically they don't know what's available nor what it's like to be in research.
And anyone who has a little bit of social skills can talk a supervisor into letting them into an honours project. Academics all like taking on honours kids, considering it's cheap, quality work.
It's more about forming a personal connection with your supervisor than anything else. If you can't do that and convince them to bend the rules for you, you probably won't be having much fun, and probably shouldn't be doing honours.