The world needs a meta-manifesto.
Manifestos are a recipe for creating one sort of conlang. As soon as you depart from that sort of conlang, the manifesto doesn’t apply. (Here is a fully worked example)
If a conlang author hasn’t explicitly subscribed to a manifesto, then it’s principles don’t really apply.
A conlanger shouldn’t subscribe to a manifesto just because there seem to be other people subscribing to a manifesto. Manifesto thinking cripplingly narrows your creative options. The best ideas and conlang features aren’t even rateable on any scale implicit in an existing conlang manifesto.
A conlang is not likely to measure up to all or even many of the various manifestos. Do we really need people to gripe about what a poor loglan toki pona is, what a lousy auxlang Elvish is, or how unnatural it is for Esperanto to lack a proto-Esperanto with plausible fictional sound changes? I think we don’t need the unnecessary harshing anymore than we need to run around a bus informing the ugly bastards that they are, indeed, ugly.
If a conlang reader subscribes to a particular manifesto, he should be polite and let people know, as in, “Your conlang, whether you intended or not, is a good example of a Naturalistic Conlang as described in some manifesto I read online.”