That license doesn't allow modifications, which is what pull requests are. The forking thing is only about making copies, not modifications.

Not a lawyer, but it seems like it could be implied-in-fact that you're allowing people to submit pull requests if you publish on GitHub. https://en.wikipedia.org/wiki/Implied-in-fact_contract

I suppose you can modify the code, but not use it (compile) as such?

Uh, so I just realized how we are discussing how developers submitting pull requests to this project with this license are basically demonstrating publicly performance art style that they've broken copyright law. Or we give the benefit of the doubt and assume they are not testing their changes at all.

Neither copyright law nor the license allow modification, so probably not.

In this specific case you might be correct but in the general case this is not true. The uploader agreeing to something does not affect the rights of other authors than the uploader.