It's an opinion that is factually wrong. Certain aspects are genetic, others aren't. An athlete with a lower type 1 muscle fiber count can become quicker just as much as one with a higher count can - through proper training, both physical preparation and technical improvements (footwork, in this case). They will only fall behind if the player with better genetics for this particular task is training similarly well. Endurance is exactly the same - some athletes will have more of a gift for it than others. It also depends on the pacing of the match, and that's where tactics, technique and mental discipline come into play.
Other aspects can also play a role in how fast a player is around the court, for example being technically better and more versatile, not having to get behind the shuttle as much as a more pan-handle-y player....
People tend to pretend that everything mental/cerebral can be coached, when it cannot necessarily, and everything physical is set in stone, which it isn't. Every athlete is unique, limited in different ways, and predictions about potential improvements are always guesswork. Tactical awareness and good decision making can not be taught to all players, just as not every player can reach the same explosive speed.
Also, coaches are not only there to refine tactics. Good coaches will either have the knowledge of how to physically prepare their athletes or delegate it to someone who has. Usually, the most efficient setup at elite levels tends to be a work split between badminton coaches focusing on technical (both racket and footwork skills) and tactical aspects, and more general athletic coaches focusing on the purely athletic aspects. However, never is a coach limited to just tactics (in the training environment). I've never seen that and I really doubt we'll ever get to that point.
