Not only can they do it but some companies have done it several times. Look at Oracle: there's HotSpot's C2 compiler, and the Graal compiler. Both state of the art, both developed by one company.
Not unique. Microsoft and Apple have built many compilers alone over their lifespan.
This whole thing is insanely subjective, but that's why I'm making fun of the "unsubstantiated claim" bit. How exactly are you meant to objectively compare this?