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So the question of how Morton Smith interpreted the singular reading, which he published, is germane. According to his published statements, the most evident interest that Smith had here related to the concept of private teaching, which could have led him to like the singular reading that he emphasized.
I haven't found any evidence that Smith knew about the plural reading (maybe someone else has). It's possible that he did, or it's possible that he did not. But that's not necessary to resolve in order to see the relevance here. If the plural reading was the text of the Mar Saba letter, then this complicates a hypothesis that Morton Smith was behind it somehow.
Of course, a hypothesis that Morton Smith nevertheless was behind the Mar Saba fragment's authorship somehow, even though he laid this emphasis on its interpretation in his written work and even though the Greek text of the fragment said something else, could still expand to include this data. In the words of Michael Goulder, though, we should fairly comment on such a hypothesis (Luke: A New Paradigm, p. 5):
It is almost infinitely elastic, and is virtually unfalsifiable.
The Secret Gospel:
We have stories of rabbis who gave obscure answers and then explained the true ones in private to their disciples. (p. 84)
Therefore he pretended that Jesus had taught them privately. (p. 85)
Even his disciples are said to have misunderstood and to have asked him privately for explanations. (p. 86)
This new understanding of the early Christian group and its private practices explained a great deal that had been puzzling in early Christianity, but it also raised new problems. (p. 115)
Another trace of the secret Gospel probably appears in the work of a gnostic named Theodotus who wrote in Egypt during the 160’s: Clement quoted from him a report that Jesus taught the disciples, "at first by examples and by stories with hidden meanings, then by parables and by enigmas, but in the third stage, clearly and nakedly, in private.” (p. 142)
Clement of Alexandria
"It is precisely from these apostles and Paul that Clement claims his teachers had received, evidently by private tradition, and handed on to him the essential teachings of the Lord." (p. 29)
"This must have influenced the way Christians looked for their own **philosophy" from the beginning, and every new, private interpretation given by individual Christians or gnostics could be justified only by saying that it was not to be found in the previous tradition of the Church because it had been kept secret." (p. 38)
"... comparing his private excerpts from Theodotus with his attacks on the Valentinians in the Stromateis; ... "this whole complex of concerns—the inner circle of higher initiates, the secret apostolic tradition, the opposition of gnosis to mere faith, and so on—is overshadowed by questions of church discipline and organization. ... (pp. 80, 81)
"In the battle over gnosticism which raged throughout the second century and reached its climax at Clement's time, private letters played an important role." (p. 81)
"Finally, the Carpocratian version of the secret Gospel had an account of Jesus’ teaching a favored disciple the mystery of the kingdom of God privately and γυμνὸς γυμνῷ." (pp. 83-84)