It appears that the T&Cs dated 10/21/2019 only preclude the redemption of rewards on Best Buy gift cards. I don't find any language disallowing purchase of 3rd party gift cards.
Except for Non-Qualifying Redemptions identified below, you may redeem reward certificates when the purchase value (excluding tax) exceeds the reward certificate amount. The following purchases or transactions are excluded from redemption of reward certificates (“Non-Qualifying Redemptions”): Prior purchases, leased products, purchases/rentals at rent-to-own locations inside Best Buy Stores, purchases from a Best Buy Store on a third-party website or marketplace (such as Amazon, Google or eBay), some purchases at stand-alone Pacific Sales® Kitchen and Bath Centers, LLC stores, or at Pacific Sales locations within Best Buy Stores, purchase of Best Buy or Magnolia gift cards, point of sale activation (“POSA”) cards, purchases made for resale, payment on any My Best Buy Credit Card, certain premium major home appliances and home theater products, any online transaction that contains the purchase of a Best Buy gift card, sales tax, service charges, and discounted Best Buy Business or commercial purchases. Other exclusions may also apply.