I get that "landing a prod diff" means "get stuff in production"? I never read this before. Is this slang unique to meta?

Nor do I know what an "eval" is, or which of the no less than three different deacronymings of "PM" (that I know of, thus far) FB uses or what that role would mean to them.

Yes. “Landing a diff” is very meta-specific.

Diff is Phabricator terminology. A diff is roughly equivalent to a Pull Request in GitHub.

Thank you!