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I am also curious what they mean parasitic leakage inductance losses.
What I can recall from leakage inductance; that it is uncoupled inductance that will ring with parasitic capacitances which will produce severe ringing. That ringing can generate a voltage breakdown in switching semiconductors, and must be snubbed. The snubbers, at least the simple ones, will dissipate that energy as heat in a resistor. Which of course is a power loss which reduces the efficiency.
But those are snubber losses. Transformers and inductor losses consist only of core or copper losses. And each has its own sub-losses: i.e. copper may divided into RDC, skin effect, fringing, etc.