Sadly, I think you're spot on. If the bud rot hasn’t spread far yet, you can remove infested buds and leave the rest of the plant intact. If you detect multiple moldy buds, it’s probably best to get drastic and remove the whole plant, root and branch. The sooner you act, the better.
Be very careful when cutting away infested buds. Make sure your fingers or cutters don’t spread spores to other parts of your plant, or – worse – to unaffected plants or another grow tent!
In the future, I'm sure you know you need to make sure your ventilation is much better and your humidity lower, especially in flower...
Sorry, mate!