Almost everything declared invalid has a link back to a specific discussion in which they were declared invalid. The functional definition of "invalid source" is that it's one which doesn't get covered here, and which may not, therefore, be used as a source for in-universe articles.
You're asking for a specific excerpt, but one's not really necessary, is it? To be declared invalid means that we don't give it attention. "Not covered", "invalid", "outside our fences", "beyond our remit" — these are all just synonyms. Surely our rules don't have to provide a thesaurus section to be clear, do they?
Do you really think that casual readers will care why we're declaring certain stories outside the DWU? I mean, how many truly casual readers are making their way to Dimensions in Time or Exile?
I'm of course interested in making things clearer, but I'm struggling to imagine a clearer construction than "You may consider this part of the DWU. But we don't". That's pretty direct, and I actually think most people will be fine with that. In fact that construction has been used for about two years and it's never even been remarked upon until really this thread.
So let me ask you: what would be clearer? Should we add a little template at the end of these articles that more thoroughly explains our position, kind of like what's been done at Worlds in Time? Would that alleviate your concerns?