You don't need to buy it on steam to launch it through steam. Just select non steam game in steam.
I was able to launch it through steam without buying it on steam. There is no need to buy the game twice except for ease of use on steamdeck ( less steps to launch it ).
You can "launch" anything through steam but if you don't buy it on steam you are essentially just clicking on a shortcut on your original .exe without any influence from steam. No reason to do so since there is no benefit
There aren't many reasons to buy the game on steam anyways tbh, and ANet makes less money if you do. You can only link one account per steam account as well, and I know most of the players who still play a lot probably have multiple accounts by now.
Main reasons are if you just want to play one account, it auto auths you, and if you have steam cash you can use it to buy the game and eotn/mission pack.