In the olden days, AC allowed easy voltage conversion with a transformer. The transformer converts the electricity into magnetism in a steel core, and then back into electricity at a different voltage.
This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields...
Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter.
Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency.
However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.
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.)
This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields...
Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter.
Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency.
However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.