How did they get to be seniors in their companies? They didn't give the title to themselves, so someone must value their skills. Is having the title mutually exclusive to being humble or self aware?
Some years ago they introduced job titles at the place where I work. My boss asked me: which job title do you want? I said: senior developer. Her said: ok. Master of the universe would have passed too.
Same thing happenend to me. I went for "Grand Poobah of Software Engineering". The boss said ok, but I think the printed business cards ended up with senior developer.
As an aside, a friend got listed with the title cosmocrator (ruler of the universe, when translated directly from greek) in the phonebook. All it took was for him to request it when they asked for a title, and, when asked what it was, reply that it was a form of kinesiology.
All in all, you probably should not but to much stock into titles. Even less so, if they are not protected.
LinkedIn is full of people who have mysteriously sprouted a Senior prefix that they definitely didn’t have when I was at the same company at the same time. So it probably is self-granted in most cases.
It is startup with little experience, so if you act confident and never go against leadership they will think that you know everything. There were also people with 0 experience and 1.5 is significantly more then that.
I am confident in claiming that title was dependent more on confidence then skills. You got it basically for never admitting you don't know something.