I don't know, I hadn't heard of this theory before.
You might want to read Wikipedia's Rāhula -- Other traditions.
It says that the Pali tradition is that ...
Rāhula is born on same day Prince Siddhārtha Gautama renounces the throne by leaving the palace, when the prince is 29 years old, [...].
... and ...
Other texts derive rāhu differently. For example, the Pāli Apadāna, as well as another account found in the texts of monastic discipline of the Mūlasarvāstivāda tradition, derive rāhu from the eclipse of the moon, which traditionally was seen to be caused by the asura (demon) Rāhu. The Apadāna states that just like the moon is obstructed from view by Rāhu, Prince Siddhārtha is obstructed by Rāhula's birth. The Mūlasarvāstivāda tradition relates, however, that Rāhula is conceived on the evening of the renunciation of Prince Siddhārtha, and born six years later, on the day that his father achieves enlightenment, which was during a lunar eclipse. Further credence is given to the astrological theory of Rāhula's name by the observation that sons of previous Buddhas are given similar names, related to constellations.
Mūlasarvāstivāda and later Chinese texts such as the Abhiniṣkramaṇa Sūtra give two types of explanation for the long gestation period. [etc.]
Maybe that's related to what you're asking about, i.e. it's some theory about how Rahula could have been born six years after the Buddha left the Palace.
I'm not sure that's a mainstream or an important "Mahayana" doctrine.
If only you knew which school to ask about (e.g. Mulasarvastivada or Dharmaguptaka) then you might be able to ask and answer, "What did or does this school say about Rahula?"