If source is publicly available, it is open source. Plain and simple; regardless of your legal ability to copy or modify.
Definitions from Oxford Languages
open-source
denoting software for which the original source code is made freely available and may be redistributed and modified.
Emphasis on "may be redistributed and modified"
Definitions from Oxford Languages
open-source
denoting software for which the original source code is made freely available and may be redistributed and modified.
Emphasis on "may be redistributed and modified"