After watching the doc Counterpunch- does Russian culture produce better fighters, or is it a flaw of the American boxing system?

I loved the Netflix doc, and it brought up some really interesting points and perspectives on the world of amateur and professional boxing.

I believe one of the boxing presidents was discussing the decline of professional boxing in America, and said something like, "If you want to go against someone from Russia, it's gonna be a sad day for you."

Is there hard evidence of Russia and/or other nations superseding the US in the quality of their fighters, or is this purely anecdotal? If so, what can it be attributed to?

This is not a Russia vs US debate- I'd simply like anybody more knowledgeable than myself to chime in (with, hopefully, supporting evidence).

Thank you