B Corp certifying makes a critical distinction for consumers. Is it a panacea for climate crises? Of course not. Will it single-handedly fix profit-incentivized business models that exploit natural resources? Of course not. It’s a start, and you shouldn’t assume that all companies undertaking its certification are merely greenwashing.
You could try to make the argument that enabling greenwashing is detrimental to them environmentalism effort, but then I’d have to get a sense for what you think is a meaningful step toward adjusting business practices.