So you have no alternative theory, but you’re just assuming Apple are lying about their reason because they won’t share their company data that led them to that conclusion?
You said they are just saying it’s poor sales, but it actually isn’t the reason. That would make their posted reason a lie.
If marketing is not based in reality, yes, it is lying, and there is nothing toxic about calling it what it is. There is plenty of dishonest marketing out there, and some of it is even legal. But I would hardly call a discontinuation announcement “marketing” since you’re literally doing the opposite of trying to sell them the product. You’re telling them they can no longer buy the product.
Could partly be that they don't want to support the development effort. Fewer phone sizes to support means they can get away with fewer staff working new versions of ios.