The Negro Units did not perform at a single arts center. They performed throughout cities in the United States. It’s not even clear there were designated centers for them when they performed.

This best captures the meaning, which relates to the way in which Negro Units were far-reaching and focused on African American subjects/audiences.

We have no reason to think this is the meaning of the phrase, because there’s nothing about the mere establishment of a federal program that’s impressive. The author is highlighting the achievements of the Negro Units. It wouldn’t make sense for the author to point out how they came closer than other theater groups to founding a federal government program. It’s not even clear that this makes sense, since the federal government founded the FTP. So how could the Negro Units come “closer than any other group of African American Theater artists had come before to founding” a federal government program?

Nothing in the context leading up to the phrase suggests there was some established set of plays endorsed by scholars (”canon”).

Nothing in the context leading up to the phrase suggests there was a collection of black-owned playhouses. The Negro Units performed in different cities; this doesn’t imply black-owned playhouses.