You're right though, the Phoenix BIOS was what made clones very possible. And it was constructed just as you stated.
“IBM PUBLISHED the BIOS code “
Yes, they did. It was part of their plant to “poison the well” of engineers that could reverse engineer a better and compatible one. Unfortunately for IBM several key players found means to not read the BIOS source code and instead used the BIOS interface specifications itself to devise what the IOS must do and let the black box guys write a new, and sometimes better, system.
It was all fun and games and people made money. Good money, too.