I chatted with Google Express. Here is the reply I got when I told her/him that GHM shows up on Walmart.com for pre-order but not on Google Express/Walmart. So now I have two different answers. Never fails!
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Unfortunately, there is a bug in the Google Express system right now that is preventing shoppers from pre-ordering the Home Mini through us. However, so long as your Home Mini is purchased from us Walmart or Walmart.com, is activated by January 15th , and you link your Google Express account to your Walmart.com account through the Google Home app, you will still receive the $25 coupon.
The $25 coupon is not for the purchase of the Google Home, but rather is for your next Google Express Walmart purchase.