Devi bhagvatam also says that brahmanas should eat prasada from Shaalagraama and worship vishnu mantra, but at an other place in the devi bhagavatam it is said that all dvijas are shaaktas, not vaishnavas and not shaivas because they worship Gayatri.
In my version of Bhagavata puraana there is a mahaatmya from skanda puraana for the bhagavata puraana and there is also a mahatmya from skanda puraana in my dewi bhagavatam.
Is it possbile that Vyaasa wrote devi bhagvatam because Vishnu gave his blessings to Durga when she helped him by becoming Shree Krishnas sister?