Indeed, as Pero pointed it out, it seems he meant the unity of Tibetan Buddhist Schools, especially the oneness of Madhyamaka, Dzogchen and Mahamudra, which has been stated long before by former Tibetan masters, like the 3rd Karmapa in his Aspiration Prayer:
"Free from mental fabrication, it is the great seal, mahamudra.
Free from extremes, it is the great middle way, madhyamika.
The consummation of everything, it is also called the great perfection, dzogchen.
May there be confidence that by understanding one,
the essential meaning of all is realised."
Interestingly, it was Sakya Pandita who criticised Dzogchen and (Gampopa's) Mahamudra, also separating Madhyamaka meditation from Mantrayana. Like here:
Even if they meditate the Great Seal,
they cultivate in meditation only a restriction of conceptual thought,
while they do not understand the Great Seal to be Gnosis
derived from the two processes.
The Great Seal meditation of the ignorant, it is taught,
usually becomes a cause of animal birth.
If not that, then they are born in the realm lacking even fine matter (arupadhatu),
or else they fall into the Disciples1 cessation.
Even if that meditation may be excellent,
it is no more than a Madhyamaka meditation.
The latter meditation, while very good in itself,
is nevertheless extremely difficult to accomplish.
As long as the two accumulations
have not been brought to completion,
that meditation will not be perfected.
To complete the two accomplishments for this,
it is taught that "innumerable aeons" are needed.
Our own Great Seal
consists of Gnosis risen from initiation
and the self-sprung Gnosis that ensues
from the meditations of the two processes.
Its realization will be attained in this very life
if one is skilled in the techniques of Mantra.
Besides this, the Buddha did not teach
the realization of the Great Seal otherwise.
Thus if one is interested in the Great Seal,
one should practice in accord with Mantra Vehicle texts.
No substantial difference exists between
the present-day Great Seal and the Great Perfection (rDzogs-chen)
of the Chinese tradition, other than a change
in names from "descent from above"
and "ascent from below" to "Simultaneist" and "Gradualist."
The appearance of this kind of religious tradition
came about just as the Bodhisattva Santaraksita
foretold to King Trisong Deutsan.
Listen, as I shall explain that prophecy. He said,
"O king, no Indian non-Buddhist doctrine will appear here
in your kingdom of Tibet because
master Padmasambhava has entrusted it
into the protection of the twelve protecting goddesses.
Nevertheless, a schism into two systems of doctrine
will occur because of certain interrelated circumstances and omens.
At first, after I myself have passed away,
a Chinese monk will appear and teach a path
of simultaneous enlightenment
called the White Self-Sufficient Remedy.
At that time, invite my disciple,
the great scholar Kamalasila, from India.
He will refute that.
Then let all the faithful
practice according to this system of doctrine."
Later, everything came to pass just as he had predicted.
After the Chinese tradition was suppressed,
that of the gradualists was made to flourish.
Still later, the royal rule itself vanished,
and some, who based themselves solely
on texts of the Chinese master's tradition,
changed the name of his system secretly
to Great Seal (mahamudra). The present-day Great Seal
is virtually [the same as] the Chinese religious system.
The Great Seal that Naro and Maitripa espoused
is held to consist precisely
of the seals of Action, Dharma, and Pledge,
and of the Great Seal as expounded
in tantras of the Mantra system.
In his Caturmudra, Exalted Nagarjuna himself also asserts this:
"If, through not having known the Action seal,
one is also ignorant of the seal of Dharma,
it is impossible that one might understand
even the name of the Great Seal."
King-of-tantra texts and major commentarial treatises also prohibit
the Great Seal to one who is unconnected with initiation.
If one realizes the Great Seal that is Gnosis
risen from initiation, only then does
one no longer depend on all signful efforts.
A Clear Differentiation of the Three Codes, p. 117-119
"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)