It's a rather intriguing concept, because it can be the case that the binaries Google released in QPR1 and their source code are different in some way. OEMs must ship QPR1 as Google released publicly within 6 months.
If this open-source release was to contain new patches, they must now ship these changes within 6 months. The Pixel OS release counts as the first 6 month timer. The source code release, by definition, now counts as the 2nd timer.
I expect the closed source binaries and public source code to be the same, but that may not always be the case. So OEMs are expected to at least in 6 months ship an update with the open-source code.
If this open-source release was to contain new patches, they must now ship these changes within 6 months. The Pixel OS release counts as the first 6 month timer. The source code release, by definition, now counts as the 2nd timer.
I expect the closed source binaries and public source code to be the same, but that may not always be the case. So OEMs are expected to at least in 6 months ship an update with the open-source code.