That is true but one loses the airdrop value that purchasing directly from the Splinterland site gives when purchasing from non Splinterland marketplaces. Packs that are actively being sold on the Splinterlands site have historically always been cheaper to purchase on 2nd/3rd party markets. This is because of many factors such as bulk purchase bonuses that lower pack cost and opportunities with the fluctuating DEC USD value. Once sold out on the official site packs usually take a jump upwards in price on all other markets or at least historically they have.
Edit: its also true that at some point each and every sold edition open value was worth less then the contents inside. Don't look at the cards in today's value. Think what they could be worth once out of print. My collections USD worth has mainly come from openign packs that had contents in them (at the time of opening) valued less than I paid for the pack. The same mindset should be applied to the undervalued Reward cards currently in print. Good luck on your Splinterlands journey.