1. Backend Engineer = Rails
2. Front End Engineer = Vue
3. Infra Engineer/Devops = AWS
...and with a security background.
...and all 3 at an expert level.
I've worked with a lot of bright and capable people over the years but I can't think of anyone matching this level of expertise.
If I had to guess; did the project grow organically for a few years where the original developer learned all of the skills as they went and now this person left?