For new memberships only (RESTRICTIONS: Valid for new Costco memberships only. To qualify as a new member, an existing Costco membership must be expired for 18 months or more.). Valid through the end of April.
You could try it, but even if the third party who is doing fulfillment on this lets you buy the Costco “activation certificate”, that doesn’t mean you will get a statement credit from Elan. I would suggest somebody who was going to buy Costco anyway try it with the caveat that you probably won’t get the credit. But Elan takes months to actually reward the credit so that info will come too late to help the rest of us.