As you have astutely observed, this goes both directions - since it is my opinion is that D&D without a DM is D&D, it must necessarily be the case that I think people who think it isn’t are wrong. That’s why I call that stance out as one-true-wayism, as it is implicitly limiting to what can be considered D&D.
I’m not saying you aren’t allowed to have a different opinion. I’m saying that the opinion you are expressing limits D&D to what you consider to be D&D, which is the definition of one-true-wayism. If that is not your opinion, then you are not expressing whatever your actual opinion is very clearly.
It doesn’t matter. A game can’t be D&D and not be D&D at the same time, so if you say it’s your opinion that a game is not D&D, the “no matter what you say” part doesn’t need to be stated. It is the only logically valid conclusion to be drawn from your statement. If I “this is D&D” and you say “no it isn’t”, you’re saying that I’m wrong. We can certainly disagree about whether or not it’s D&D, but it’s my opinion that your stance limits what can be considered D&D based on your criteria. To one true way, you might say.
No, there’s a true answer. A game is D&D, or it is not. We are disagreeing on whether a particular game qualifies as D&D, and my opinion is that your view is too exclusionary - or in common parlance, one-true-wayist.