Your link says "payment must be made by" (emphasis mine) and lists a variety of forms, but does not specifically list the TCU card as an eligible form of payment for target.com. Therefore, TCU is ineligible on target.com for the employee discount, even though it's accepted in store.
"***At this time, the Target Credit Union Credit/Debit Card is eligible for the 10% Team Member Discount in stores only and not on Target.com. The Target Credit Union Credit/Debit Card is not eligible for any of the REDcard offers, including the 5% REDcard Discount and REDcard Free Shipping at Target.com."
From the Team Member Discount page on eHR. Last updated Aug. 2017. (emphasis mine)