He’s explaining this is “working as intended”. Sometimes two mechanics work in a way that hurt the player, we can call it “anti-synergy”.
The game doesn’t have two different “dies”. So when the Sunbird uses its ability, its “die” is the same “die” as if something killed it. There is, in the game “stack”, the notion that it should return. But since there is a “when an ally dies” trigger, that happens first. Then, there’s nowhere for the Sunbird to go, it can’t resurrect.
For this to not be true there’d either have to be two “dies” so the game can tell the two apart. Then, all “when an ally dies” traits would not work with Sunbird.
So really, the devs would have to special-code Sunbird and every “when an ally dies” trait in the game and figure out which ones players want to work and which ones they don’t. That’s a lot of work. And it’s intuitive to point out that “dies” means “dies”.