For context, Walker wasn’t cagey in the slightest about the character’s future, and spoke as if Brainwave Jr. is no more.
In fact, the actor got a little more gruesome when offering his thoughts on his character’s current whereabouts.
Stargirl wouldn’t even need to introduce this particular reveal right away, since there are already lots of other story threads to weave together.
It could easily be held to become an arc in Season 2, which would be why Jake Austin Walker wouldn’t necessarily be aware of the storyline’s existence at this point.