This article is generally positive. The author liked the people, had a positive sentiment of the interview process, was impressed by the tech stack, had a solid manager, etc. It just wasn't the tech stack they wanted to be working with.
People don't want to hurt their employment prospects, so it's understandable if they are selective about what they reveal, but that's why I don't trust accounts like this.
I left HashiCorp in 4 months for basically the same reason, just swap our frontend with Go.
I was interviewed in Go and I knew the team was using Ruby but all the teammates were energetic for having someone else who knew Go, wasn't all about Ruby/Rails and the team was great. But the opportunities on other teams to build out things in Go and work on the existing Go codebases, both public and private, was just teasing me every day. I tried to work on one of our open source libraries and was told I was stepping on someone else's toes and could be taking opportunities away from them.
I would return in a heartbeat on one of the Go teams working on one of the Go products. I think they are just great in the Go community and their tools are fantastic.
edit: this wasn't meant to insult OP, sorry
Why is there always the tendency to blame the person and NOT the company?
If you apply for a role that is explained to you, what does it matter what the company mostly does? If I apply to a rail maintenance company as a programmer, should I expect to maintain rail? No.