You can be effective without a well tunneled slider, obvs. Just weird that the article is talking about how Gallen checks the tunnel box, which is something that the sox look for, when Gallen doesnt have a slider.
Most of the time, when people are talking about tunnelling it means that the slider looks like a fastball out of the hand for a long time because it stays in fastball shape for longer (late-sharp break on slider). For a well tunneled slider, you have to look for the dot on teh ball, and not the shape, because the slider shape doesnt reveal until after you have to make your swing decision. Its not accurate to talk about tunnelling a curveball or a sweeper becuase thats not what they are even trying to do (look like a fastball until last second).