Miro Senior Full Stack Engineer interview questions

I am a tech lead and applied for the Senior Full stack Engineer position. the only positive thing about their process is the recruiter call, he was very friendly. next step is a coding challenge. A common thing with all their interviewers is that they are extremely unfriendly, they gave me a leetcode question which was very easy I finished in 10 mins and optimized. I was surprised when they said that the interview is over I thought I made a mistake or something because it was really short but I actually moved to the next round. next is the system design. again extremely unfriendly interviewers. The question asked was actually a cool question and I was excited to start designing then the team lead stopped me and said we will only focus on the frontend and "assume the server is a black box", that should be a criminal offence to say that in a system design interview, so basically this is a frontend design interview. My assumption is that they had a frontend engineer interview me and they had no knowledge of the backend so they were uncomfortable with asking questions their.