This is a fantasy baseball list and the rankings are based on who is likely to help a fantasy team in the near future. Greene has a couple more years of minor league starting before he'll be ready for a big league rotation and innings are the primary reason. Greene is absolutely a prove it prospect because he hasn't really pitched a season where some innings have accumulated on his arm since he was a Junior in high school. He needs to prove he can handle a full season of starting and it's probably going to be too soon for him to do it in 2021. Still, he's the team's top prospect IMO, just not from a fantasy perspective for this year.
I do think Greene could contribute this year if the Reds wanted to use him in the bullpen. The team's pen is thin and using Greene would allow the reds to address that without spending money or having to wait for Greene to build his innings. If the Reds were planning something like that, he'd probably be in the top 10 on this list.