This is not true. I'm not wading in to say one company is better than the other. I'm saying I've tried a lot of discs from a lot of different brands and I haven't found anything that flies quite like a Teebird. Maybe some Legacy Rivals. Nothing that Trilogy has though.
Same with Firebirds. Sure, other companies have speed 9ish overstable stuff, including Trilogy, but they don't fly like a Firebird. The differences are important.
While I appreciate you opinion, I think this has become more urban legend than truth. I can't argue that there is no disc identical to a Teebird or Firebird, I am sure this is true. But, I certainly don't need any disc identical to them. Why would anyone try. There is no magic in either disc (yes, I know this is heretic speak). If I really want a flight exactly like either, I should throw them. The subtle difference are VERY important, and why many would throw something else. I am now not sure if we agree, or disagree, lol.