If this were true, and initial candidate retrieval were a solved problem, teams where search is revenue aligned wouldn't have teams of very well paid people looking for marginal improvement here.

Treating BM25 as a silver bullet is just as strange as treating vector search as the "true way" to solve retrieval.

I don't mean to imply that it's a solved problem; all I'm saying is that in a lot of cases, the "weak initial retrieval" assertion stated by the article is not true. And if you can get a long way using what has now become the industry standard, there's not really a case to be made that BM25 is bad/unsuited, unless the improvement you gain from something more complex is more than just marginal.