While machine language is a software concept, as a hardware designer, you still need to be familiar with it and how it is used to integrate with the devices you design. During the interview as a senior hardware engineer, you may be asked questions that are peripheral to your main occupation. This is because you have to collaborate with stakeholders outside of the design team when developing hardware or equipment. Being familiar with the terminology they use, some of their considerations, and the business objectives they are trying will help you do your job better. The interviewer seeks to understand your abilities in this area.