If the sequence diagrams were comprehensive enough, the design was based on well-known design
patterns and the name of the classes were descriptive enough, you could guess what the class diagram would be, but you may not get 100% accurate
It's just like trying to figure out what the parts of a clock are by watching it work. It's classic reverse engineering. You look at the behavior and try to design the thing yourself. You can get a good working model, but your model may not reflect reality 100%