> as no one can write code for memory training to bring up the chip

Surely someone can do it, but it's probably too niche to do. The licensing fee is probably cheaper than corporation spinning the board and reverse engineer it and for hobbyists lower tier memory likely was fine.

That said given that such technology has become so much more accessible (you can certainly create FPGA board and wire it up to DDR4 using free tools and then get board made in China), it's probably a matter of time someone will figure this out.