TIOBE index is a junk metric (I'm surprised so many technical people seem to buy into it; it's the worst kind of easy to get a precise number, hard to demonstrate that that number has any more than a very distant relationship to anything meaningful, kind of metric that, IME, tech people mock bad managers for buying into.)
That said, Ruby’s got (via Rails and other web frameworks, and via various testing and admin tools) a very strong footprint in use even if it's no longer the hype-of-the-month.
It’s popularity at this may have diminished with the rise of JavaScript, Go and alternatives but many folks still like and use Ruby, or at least would like to see it continue to improve and thrive.
Similar to general hiring/salary surveys, I’m not sure that the referenced index is accurate.
It also has an amazing testing framework in the form of rspec, which can be used for much more than just testing Ruby code.
TIOBE has it's issues but as a rough guide to what the industry is actually using I think it's in the ballpark.