No, until now, you were not allowed to fork just because you agreed not to install Google apps on that phone -- you were only allowed to fork if you didn't want to install Google apps on any phone you made. So, for example, Samsung couldn't decide to offer an Android phone without Google unless it was willing to shut down its entire current business.
The allegation is that because of this, anyone looking to fork Android had a very hard time finding good OEMs to make their phones. E.g., maybe the Amazon fire phones would have worked if they could have gotten a Tier 1 OEM to make the phones so they could have put out a better product. But nobody would agree to stop making all of their current Android phones to see (I assume; there is no public version of the decision yet so I'm making up the example).
AOSP is completely open for anyone to work. It's as simple as that. Nobody is forced to use Google apps. They are free to fork and come up with a way for people to install apps from their own app store.
The allegation is that because of this, anyone looking to fork Android had a very hard time finding good OEMs to make their phones. E.g., maybe the Amazon fire phones would have worked if they could have gotten a Tier 1 OEM to make the phones so they could have put out a better product. But nobody would agree to stop making all of their current Android phones to see (I assume; there is no public version of the decision yet so I'm making up the example).