What's really funny is that particular quote is also my response to you.
Yes all supports can build the same actives, but building RG and Shurima on Janna is a much more versatile support than RG and Shurima on Thresh. The best part of that example is that Thresh is the next best swiss army knife to Janna. If you were to use that example on say Annie, the contrast just becomes more stark.
Items are important when considering a champion's strengths since the same item does not interact with every champion of the same class identically.
But in any case, if you play Janna, you are expected to know how to catch people with Q and W and help lock them down. Janna is most definitely the queen of peel, disengage, and kiting, but if you think that's the only thing she's good at, you're simply deluding yourself. Let's go with the simplest example. Build captain + mobility on Janna. Your team is suddenly travelling around the map mid game 18% faster. That changes everything because now she doesn't need to Shurima as often (even though she can activate it to get to lane faster and have it be up when she reaches the tower) and can save it for more extreme circumstances.
Can any other support do that? Sure they can build captains + mobility and help with the engage, but that's only 10% more MS. That's not enough to be able to dial back on Shurima or RG as often. But 18%? Now you're starting to creep into that territory. That is just one example of how the multiple uses of her spells allow her to specialize her spells and remove some of the meaningful choices she has to make.