Please read the original article. Nowhere in it (or the original talk) does Alex advocate using Scala instead of Rails for the things that Rails does well -- namely, interactive web page rendering.

Within that niche, Rails does a fine job. For infrastructure and middleware code, however, Rails has little to offer other than the Ruby language, which has serious issues when applied to time + memory-intensive problem domains.

I'm a huge fan of Ruby, and a (perhaps somewhat begrudging) user of Rails, but that doesn't mean I think they're the right solution to every problem.