The theory in the article is that Apple is intentionally making the older devices slower.
The theory you are responding to is that Apple is intentionally adding new features to iOS, which incidentally makes older devices slower because they're trying to do more work.
Is it a "conspiracy to make your device slower" when Apple adds new features to later versions of iOS? They're not intentionally adding in "slow down a bit here and encourage the user to upgrade to a new phone" code. So there's no conspiracy.
The theory you are responding to is that Apple is intentionally adding new features to iOS, which incidentally makes older devices slower because they're trying to do more work.
Is it a "conspiracy to make your device slower" when Apple adds new features to later versions of iOS? They're not intentionally adding in "slow down a bit here and encourage the user to upgrade to a new phone" code. So there's no conspiracy.