Yeah, it does those things well, but those are all the same role, or if you really don't think so, then those are still checked by the same things. How is that flexible, when all its "roles" are so closely intertwined? You know exactly what Fletchling is going to do when you see it: Spam Acrobatics, U-turn on a predicted switch, and maybe have a coverage move. Ponyta and Tirtouga are more flexible than it because they at least have two different sets. Ponyta can be either physical or running a SunnyBeam set, while Tirtouga can be a Shell Smash sweeper or an Eviolite tank that can set rocks up. That's what real separate roles look like, by the way. The two sets of each of these Pokemon have different goals, different moves, and most importantly, different checks. Chinchou's a great switch into physical Ponyta, for example, but gets wrecked by the SunnyBeam set. As for Tirtouga, Fighting-types make a great check to the SS set by using Knock Off + Fighting move to KO it, but the bulky set has Solid Rock and 30 Defense with Eviolite to make even Timburr's Drain Punch into a 3HKO, from there allowing Tirt to set up rocks or fish for a Scald burn. Fletchling does not have real flexibility; it does the same thing with every set it can run, it's goal is the same every time, and it's more or less checked by the same group of Pokemon with every set. To sum it up, the difference between something like Tirtouga's sweeper and tank roles are using Shell Smash and its STABs to sweep, and setting up Stealth Rock and taking hits easily; the difference between Fletchling's revenge killer and sweeper "roles" is using Acrobatics either early-game or late-game. Quite the difference if you ask me.
As for the Pawniard comparison, notice that none of its checks are switch-ins, besides Trubbish, and maybe defensive Mienfoo. Switching into Pawniard is almost impossible without taking lots of damage, and for every non-Sticky Hold switch-in, it means taking 50% more damage from Pawniard's team for the rest of the game. Pawniard is on an entirely different level than Fletchling (oh, and Pawniard has real flexibility, too!), so I don't believe that the comparison is valid.