Hear goes the best advice I, as a senior Elixir developer, can think about: chill out. 
From my experience you can’t be prepared for everything. One time my application was rejected not because I don’t had required Elixir skills, but because they found a similar applicant who knows hockey rules - yes, this offer was from Canada. 
As long as you have a job (i.e. a stable source of income) you don’t have to worry.. It’s just a matter of time when you would get an Elixir-based job. Sometimes it’s just really a matter of luck. 
Unfortunately I also need to say 2 bad things about getting a job:
- When they are reaching you (instead of you reaching them) your chances drastically increases.
- The above works especially if a message wasn’t generic and somebody really took a look at your profile.
Looks like all of above matches your case. That’s already a good start! 
If none of above would match your case then you should not worry about such job. I don’t mean not to apply, but don’t take it so serious if you are rejected and especially in case:
- there was no reason
- there was a stupid reason
- you even not received a reply, because before you have ever applied, they already found someone and therefore they have finished whole recruitment process
Well … looks like I was “unlucky” many times as all of above happen to me at least once and I simply learned to “deal with it” - no matter how bad this sounds. 
Good luck! 
