Submit other people's code together with a paper
I don't think the fundamental issue here is how to submit the code, but rather how to give credit in the paper.

How did you describe the code in your paper?  If your experiments use other people's code, then you need to indicate this explicitly and give them credit.  Even if the author is anonymous, it's important to indicate that you are not taking credit for it yourself, as well as to indicate where readers can obtain the code.

If you have already dealt with this in the paper, then you have no problem when it comes time to upload the code for review, since the reviewers will already know exactly what to expect.

If you haven't dealt with this in the paper, then you have a problem and will need to revise the paper ASAP.

As for whether it's from an untrusted source, you'll need to be upfront about this.  If you can justify why this is a reasonable choice (for example, you've checked the code reasonably carefully), then you should do so.  If you can't justify it in a way that reviewers will accept, then you have a problem, but you shouldn't try to cover it up.