It's essentially a technological breakthrough. That charger uses GaN (Gallium Nitride) instead of silicon for the switching transistors. Gallium Nitride has various advantageous properties over silicon (higher bandgap voltage, higher breakdown field, faster electron mobility) that make it more efficient in chargers.

In theory, would SiC be a further improvement over GaN?

There's a slow transition underway from Si to SiC power-semiconductors in industries such as automotive.

No. Carborundum can operate at higher temperatures and voltages than silicon, but it isn't more efficient; it just lets you run the transistors hotter and at higher voltages, so they can switch more power. The nitride is actually more efficient, but it's more delicate to overvoltage. (I think it also withstands higher temperatures, but in a wallwart the problem isn't that the transistor loses its ability to switch; it's that the plastic case melts.)

You can relive my emotions from when I learned about the astounding properties of nitride transistors (in 02017, I think) in https://dercuano.github.io/notes/jellybeans.html#addtoc_1. Unfortunately there weren't any carborundum parts in my list.

SiC has a lower electron mobility than GaN (and, in fact, even Silicon), so in identical scenarios it can't switch as quickly as GaN chips.

SiC has significantly higher thermal conduction than both Si or GaN, though, which makes it more suitable for cases where you need a ton of thermal capacity (inverters for EVs, for instance).

Both technologies are in their infancy, though. You can get better performance out of either by improving the process technology aspect, so it's hard to say one will ultimately be better than the other. Given the electron mobility though, it seems GaN will ultimately win for devices that aren't thermally limited.

I just bought one of the Anker Nanos and it's kind of amazing to charge my macbook off something so tiny. It works ok if a little slowly. (M1 air)

Some of the new ultra-compact chargers are using switching regulators based on GaN transistors, which are a fairly new technology. These are more efficient and hence can be smaller for the same power envelope. Some of the Anker products use these, not sure about the one you linked.

But similar logic applies even for the ones that aren't using GaN. More efficient parts and/or designs allowing for more and more shrinkage. One of the other big drivers has been manufacturers starting to market monolithic chips that manage all the aspects of power regulation and USB-PD in a single/small number of parts.