The author claims that, because the Protoevangelion/Gospel of Mary presents St. Joseph as a father and a widower as opposed to the ever-virgin man that St. Jerome proposes that it was rejected in the West. St. Joseph's portrayal, going against the 'official' Roman Catholic vision, was enough to get this Gospel excluded from their traditions. However many of the stories it contains made it into Western Christian devotions just the same, through variations and retellings.
In this case 'lost' doesn't mean lost and then discovered recently, but rather rejected and then forgotten by the majority of Western Christians. It has never been lost to the East.
There's been a sort of popular flood of 'lost' or 'secret' documents in the recent past: The Gospel of Judas, The Gospel of Thomas, etc. Mostly documents of Gnostic or otherwise dubious origins that are 'discovered' - despite the fact that their existence has been know for several decades if not longer - and then someone publishes a book, unveiling the 'secret' and the implied conspiracy to keep these facts from the public.
I can tell you, having recently read a fair chunk of 'lost' New Testament-era texts (mostly Gnostic) that they were 'lost' for good reason. They make sense best when one is high on something.
"The French Egyptologist Jean Doresse was one of the first scholars to examine the trove of Gnostic literature found near the Nile village of Nag Hammadi in 1945. In The Secret Books of the Egyptian Gnostics (1958), Doresse writes that these Gnostic works had been previously known only through excerpts quoted by their opponents. It was assumed that defenders of orthodoxy, like St. Irenaeus, had chosen the most ridiculous passages for critique. But 'it is as though Fate had been trying to poke fun at the learned,' because the intact works, now that they've been recovered, turn out to be 'the most complicated and surely the most incoherent that Gnosticism ever produced.' The ancient orthodox critics of these works now 'look almost eulogistic, almost benevolent' in taking them 'so kindly and seriously as to do them the honor of refutation.'"
We have the canonical New Testament on the one hand and the various non-orthodox (Gnostic and otherwise) works on the other. There is, however, a third category of works. Those that were widely circulated by the early Christians and while they maintained orthodox teaching did not rise to the level of Scripture. Inclusion in the New Testament was predicated partly on making a convincing claim of apostolic origin, and many got in only after a fight. The Revelation of St. John and the Epistle to the Hebrews were not accepted as canonical by some communities until the late fourth century.
The third category of works takes on many different forms: biographies, sermons, visions, letters, biblical commentaries, debates with non-believers, etc. The Shepherd of Hermas and the Didache are just two examples of this third type of document.
Many of these documents have been 'released' into the West fairly recently and people wonder where they've been hidden for so long. The answer is that they were neither hidden nor lost, but rejected and ignored until successive generations didn't know of their existence.