interesting. nyingma was "founded" (if that is such a thing) at the height of the abbasid caliphate. the persian world at that time was highly academic and more liberal that most would think. as I understand, most of western scientific knowledge--medicine, astronomy, mathematics, etc.--has its roots in the ancient persian world, although our history books have overwritten much of their discoveries with western names. imagine trying to do multiplication tables with roman numerals, instead of the arabic, base-ten ones we use today. al-khwarizmi, who wrote the treatise al-jabr (i.e. "algebra"), was an interesting character.
the extent that the buhddist knowledge was brought in from persian texts (rather than sanskrit) does not mean that they were not buddhist texts, since the abbasid academics would have translated and studied those texts. buhddism was widespread in the califate, and promoted in certain castes at different times. perhaps the abbisid-buhddists had their own treatises and understandings, although I suspect the parchment they were written on has long decayed, or was destroyed by the mogols. you have to wonder how much ancient knowledge was lost in the destruction of the baghdad library.
one thing is for sure, the ancient world was highly interconnected, much more than most would think. so it should not be surprising that people were sharing ideas, methods and techniques across cultures, and that for a particular school of thought, that there is usually no one, single dogmatic source for the knowledge.
the extent that vajrayana borrowed knowledge and techniques from cultures other than ancient india, does not mean that it is not buhddism, nor impinge on its validity as a course of study.