This is true, however, keep in mind that x86 is the 4th most produced CPU family, behind ARM, MIPS, and PowerPC, and RISC-V is coming up real damned fast to bump x86 down to 5th.

All CPUs today are RISC descendants: yes, even Intel and AMD's x86.

The first "not actually a CISC x86 at all internally, but 100% a RISC" was the Pentium Pro (1995, and recycled a lot of work put into the i960, thanks to Fred Pollack), and AMD's first real[1] CPUs weren't x86 but a RISC descendant called the AM29k, which when they slapped an x86 decoder on it and it became the original K5 (1996), which sometimes won benchmarks against the Pentium, especially on code that wouldn't take advantage of the Pentium dual ALU design; the AM29k/K5's OoOE was superior due to it's RISC nature.

So, although, yes, technically there is a x86 duopoly, but x86 kind of ceased to exist meaningfully in the 90s. It is a RISC descendant monopoly, and whoever is in the lead at any given moment is who has the best fab tech; currently it's a fist fight between two Jim Keller architectures (he worked on both AMD's Zen and PASemi's ARM family) both using top tier TSMC tech. Jim Keller himself is a very accomplished RISC jockey, and I absolutely respect him and any team he's been part of.

[1] Depending on how you define real; the team that left Intel after management screwed them and started AMD, reverse engineered and reimplemented the CPU that put Intel on the map, the 8080, called the Am9080, after making a lot of "bag of chips" sort of ICs that weren't CPUs. All the other CPUs between that and the AM29k were second source chips under license.