Disregard the comment you're replying to, they're either misinformed or deliberately inaccurate in their assessment.
The backdoor consisted of a combination of publicly visible code and binary blob test files available in the project's repository as well as obfuscated build scripts which were contained only in the released tarballs, tucked away, which, nevertheless, were also publicly accessible, decompressable and auditable[1]
So discovering the exploit absolutely depended on FOSS. If the vulnerability was in a Oracle product, nobody outside Oracle would have access to their tests (and publishing the bug would be a legal issue).
The backdoor consisted of a combination of publicly visible code and binary blob test files available in the project's repository as well as obfuscated build scripts which were contained only in the released tarballs, tucked away, which, nevertheless, were also publicly accessible, decompressable and auditable[1]
[1] https://gynvael.coldwind.pl/?lang=en&id=782