Well, no option will ever be the correct choice for absolutely every single option, and some strings are flat out BDash counters. Not every character has this sort of string and even less consider their best pressure string or can even continue pressure after throwing it out. It's as good a defensive option as any other backdash is, at the very least.
I mean the same can be said for just about every D1 in the game, most of them range from -7 to -11. And even as far as 6 frame D1 goes, it's not even the most punishable, Ermacs is a frame worse on block, QCs is tied with Shinnok, Scorpion, and I BELIEVE one other one I forget who but I know the only people with 6f pokes less punishable are Kotal and Goro. And sure, we can say QC gets a lot less offensively off his D1 which is true, but it also gets him out of pressure, and for a character who already has the best offense in the game which is offset by his lack of ways to get out of pressure, the way this translates to his gameplay is still very strong. It's not the best D1 in the game, but it's an excellent tool that he did not need. On the positive side, as far as 6f D1s go, Rude is right, it works as it should. Gets him out of pressure and gives up his turn on block, but doesn't guarantee anything offensively unlike say Scorps 6f D1.
But I think it's important that we recognise the character has a 9f low hitting D3 which is +15 on hit, -1 on block, aka Sub-Zeros D3. Quan never has to press D1 unless he's using it to get out of pressure, and even the only at times with gaps that are 9f or or lower - a job which it does just fine, should it really give him his offense back for free on hit as well? He shouldn't even really have the 6f D1 for getting out of pressure at all, he has much more options for getting out of offense than his opponents have for getting out of his, the only argument for him getting he 6f poke implemented in the first case was that he gets block infinite without it, except the block infinite was removed as soon as he got the D1 anyway so now the only justification is "well it isn't THAT good and doesn't get his offense started", sure but that's not his only poke and we needn't look at it in a vacuum, it works just fine for doing its role of breaking pressure on a correct read, and this character is meant to be balanced around not having such options. How you gunna have the fastest poke in the game, the fastest NJP in the game and top 3 backdash in the game, and claim NO defensive options. "No defensive options" is what his opponents have against him when they get caught in the blender. Vice versa the same cannot even close to be said