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?
I don't quite get your point though. Mine was only that it's common for single companies to develop multiple independent state of the art compilers, whereas after the 1990s video codecs tend to be collaborations between many companies. That's a piece of evidence that codecs are harder. But this is all quite subjective and I don't really care. Maybe compilers are actually harder and the trend to collaboration in video is just a cultural quirk of that subfield - doesn't really matter. The starting point of the thread was a belief that if MPEG didn't exist video codecs would have all been 100% free right from day one and I just don't see any evidence for that. The competition to MPEG in the 90s was mostly Sorensen and RealVideo if my fading memories aren't too garbled. Although the last version of Sorensen Spark was apparently a tweaked version of H.263 according to Wikipedia.
The only reason I can think of why you would say this is that nowadays we have good compiler infrastructure that works with many hardware architectures and it has become easy to create or modify compilers. But that's only due to the fact that it was so insanely complicated that it had to be redone from scratch to become generalizible, which led to LLVM and the subsequent direct and indirect benefits everywhere. That's the work of thousands of the smartest people over 30 years.
There is no way that a single company could develop a state of the art compiler without using an existing one. Intel had a good independent compiler and gave up because open source had become superior.
For what it's worth, look at the state of FPGA compilers. They are so difficult that every single one of them that exists is utter shit. I wish it were different.