See, this is why it frustrates me that the Destiny community thinks meta is an acronym for most effective tactic available. It’s not, it’s short for metagame. Metagaming is any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game. An example of this would be using a particular weapon for reasons other than its game statistics. For example, shotguns are an extremely popular metagame choice right now because maps are small and player movement speed is high, which makes them more effective for reasons beyond of their game statistics. Telesto is also a popular metagame choice because it’s an effective counter to shotguns, which again, is a factor that transcends mere game statistics.
In other words, the very reason you give for Telesto being so popular despite not being the Most Efective Tactic Available is what makes it a strong metagame strategy.