They aren't a mining company. So what about them? Companies source "Canadian diamonds" I would be surprised if Brilliant Earth could guarantee none of their diamonds are from DeBeers.
I'm just stating facts. A lot of people are confused about Canadian diamonds and assume that because they are Canadian they are ok because they have no connection to DeBeers who they see as bad. Most don't realize the level of involvement of DeBeers in the Canadian mining industry.
Even if DeBeers wasn't involved though you're still supporting and industry and an ideal that make DeBeers viable. So saying "I don't support conflict diamonds because I only buy Canadian" is wrong because its the demand you've created that make those conflict diamonds valuable.