This quite arguable. Chogyal Namkhai Norbu explains that this equation of the three inner tantras with father, mother and non-dual tantra was a political gesture. He definitely does not agree with this sort of statement.
The fact is that Longchenpa, etc. all treat HYT as "outer tantra".
Longchenpa’s [Great Chariot ] states:
Mahayoga, the father tantra, the nature of method [related to] appearance,
[is taught] to benefit those whose strongest emotional affliction
is aversion and who are subject to excessive discursiveness.
Anuyoga, the mother tantra, the wisdom of the completion phase
[related to] the essential reality of emptiness, [is taught] to benefit
those whose strongest emotional affliction is desire and who delight
in stillness of mind.
Atiyoga, the nature of nonduality, [is taught] to benefit those
whose strongest emotional affliction is delusion and who are tarnished
by [attachment to] effort.25 The Great Array states:26
The generation phase is for minds elevated [with many
The completion phase, for minds with attachment [to the
experience of bliss];
And the great perfection, for the most secret and supreme.
Finding Comfort and Ease in the Nature of Mind states:
Mahayoga emphasizes winds and the methods of the generation
Anuyoga emphasizes the constituent [of bliss] and the wisdom of
the completion phase;
Atiyoga emphasizes pristine awareness from which nothing is
The Treasury of Knowledge Book Six, Part Four: Systems of Buddhist Tantra The Indestructible Way of Secret Mantra. page 308