Citation needed on DC/DC converters being as efficient as AC transformers. I'd agree they can cost less, but AC transformers are wildly efficient. 99% efficiency is not unheard of. Full load efficiency can be extremely high too.
There are basically no DC/DC converters that hit that efficiency at any load.
Give the benefit of doubt and read the surrounding context more carefully.
Not only did they mention (rephrasing) for unlimited money / resources AC / AC transformers could be far more efficient, but the part you are critiquing is simultaneously comparing COST for SAME efficiency parts (I assume typical) and resulting efficiency for similar COST.
AC transformers have fixed thermal power density, but load power scales with mass to the 4/3rd power (i.e., scales with linear size by the 4th power, while volume and mass and loss only scale to the 3rd power).
And no, 99% isn't hard for a resonant switched capacitor converter.
They just happen to be restricted to integer voltage ratios. (With sometimes a few percent regulation around this ratio without substantial efficiency loss.)
There are basically no DC/DC converters that hit that efficiency at any load.