This is plain wrong. In the US it was assumed that Oxygen-rich preburners would erode any manufacturable material - metal, ceramic, or exotic. When the RD-170 (If I remember correctly) was found to have an oxygen-rich preburner, very knowledgeable people called it a lie, misrepresentation, or proclaimed that is was severely wearing (possibly ablating) and thus had to be very heavy.
The Raptors are reusable (even air-ignitable, with centisecond timing precision to achieve a specific thrust) engines with an oxygen-rich preburner, and to boot they have the highest chamber pressure of any rocket engine ever. Thus, they also have a very high (possibly highest, we don't know) pump pressure (the preburner _is_ the pump), and they're doing that with oxygen-rich combustion.
There is some crazy material making up that preburner chamber, and the pipes down to the pintles. Materials that until recently were though to be beyond manufacturing capability. And this is in a reusable engine that costs less than an RD-68. Frankly, it looks like magic.