It doesn't take much to make up a lineage. One can also just say that it was a transmission from one of the buddhas, bodhisattvas or vajrasattvas.
But I'm not trying to defend them just point to the phenomenon of legitimising oneself through claims of lineage and tradition. This exists not just in Buddhism but in so many other forms of doctrines (教 - can't think of a better word that covers religions, schools of thoughts and such) like neo-paganism, Western esotericism, Hinduism, etc. And all these traditions most of the time lack the kind of historical background they want people believe in. It seems to me that either a tradition dies out at some point and then it's later revived or it becomes established enough to stay alive but because of its institutionalisation lineage becomes meaningless, a mere formality and paperwork.
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)