I don't know the specifics, but the license included with that code will specify what can and can't be done with it. ~~Not all open source is free to use in your own code base. Sometimes it is open sourced for review purposes.~~
> Sometimes it is open sourced for review purposes.
Huh? I'm not aware that it's possible to open source something without, you know, making it OSS. Am I mistaken, or are we talking about two different meanings for "open source"?
Sorry, you are right. The issue I was thinking about is that there are different OSS licenses, and they have different requirements for use. Attribution and open sourcing any changes you make to the original code base are an examples.
The agreements for opening code up for review purposes only is indeed not OSS, despite the word 'open' often being used there.
I started off sound, then diverged. The part of Twitter that was open sourced will have a license spelling out what you need to do (if anything) in order to use the code. The review part is a different thing.
I assume OP was referring to “source available” schemes where the source is published or sometimes selectively disclosed but not licensed for anything other than review, ie you lack the right to run or copy it.
Needless to say, it’s not accurate to call that “open source”.
Edited: the last line is incorrect.