Yeah that is true. I'm hoping Costco has their supply chain down well enough that it's not an issue, but at least there is a slightly higher chance it's legit if it says Clorox on the side.
Unfortunately, I've never seen information about how Walmart handles this. Given their massive shift to 3Ps providing most of their products online...I'd imagine they use a similar strategy (it improves same/next day availability and lowers cost). I follow the same strategy with Walmart (try to only buy directly).
It is a sad time for online purchasing :(