There is a stark difference in the words used in sanskrit of other Parvas and the last chapters of Pratisarga Parva. This shows that those have been written in recent past. The chapters about Isha have been added with stories from Bible to make it look that Noah was Manu. It was the first great game played by evengelicals to convert entire sanatan dharma to christianity.
https://vedpuran.files.wordpress.com/2016/07/pratisarga-parva-third-khand.pdf
If you read Vishnu Purana and kalki Purana, you find no mention of these events in any other Purana. This shows that they have been added to the original texts and have corrupted the Bhavishya Purana.