Pretty much this. You can have experience and also perform well in a system design interview. However, passing some of these rounds involves chance. Some interviewers are looking for specific answers. For example, some interviewers are looking for you to be knowledgeable about recent papers specific to the heavy-hitters problem. Or the two ways to best support lynchpin objects. It’s not so much as dropping well-known building blocks to construct a system but rather the interviewer asking you to design a system requiring specific characteristics you might not have any specialization in building.