It's not that the task sequence blocks GPO - it doesn't - its that GPO doesn't run until the Windows Setup completes. Since the task sequence runs entirely within Windows Setup, then GPO doesn't run until the task sequence completes. This is documented here:
Theoretically GPOs can run during the task sequence if its triggered appropriately, but it's not recommended since certain GPOs can conflict with the task sequence. Also, this only really applies to task sequences installing a new OS. For other types of task sequences - stand-alone, in-place upgrade, GPO can theoretically run while the task sequence is running.