Just my opinion, but I believe I read an articles on foxsports where Josh Green was considering, wanting, hoping to play in the Senior Boomer's team.
https://www.foxsports.com.au/basketball/exclusive-josh-green-wants-to-make-his-australian-boomers-debut-in-the-2019-fiba-world-cup/news-story/e06b157ce8cec14bdbc9f29275200a77
If that is the case, then playing in an U19 Australian team would be, IMO, considered by him, his entourage, his management, etc, etc as beneath him and would serve no useful value as it might just show him up against other kids his own age and show deficiencies in his game and could be detrimental to his NBA draft prospects.
He's using similar tactics to Dante - not going to college, Thon Maker - not going to college nor playing for Australia in junior FIBA events.
Like Dante & Thon Maker, keeping it all a mystery has currency.
Look, good luck to him, but Markelle Fulz and others have tore up USA high school basketball and turned out to be not so great in the NBA.
For Josh's sake, I hope it turns out.