What about Fleetway indicates that it's a seperate universe from Archie? I know it has original stories/characters but as far as I know none of it contradicts Archie's continuity. Plus like more than half of the stories are Archie reprints anyway, innit?
Well, in the past, perhaps Dean Clarrain might have been able to clarify that. Though now, as Peter Laird told me in his TMNT blog's Q&A, only Viacom has the authority to determine what is and is not canon in any particular official TMNT continuity. But there is precedent for material not specifically published by Archie Comics to still be in the Archie continuity—Ninjara: Seed of Destruction, which was still written by Dean Clarrain and Chris Allan but published in a 1990s fiction magazine for the furry community.
Here's the Dictionary.com and Wiktionary entries for "storybook". Basically, it's a book of children's stories. It's also used as an adjective, meaning something that is pleasant and idealized, as in a "storybook ending".
In my life experience, it's mostly for children. Basically, if a child asks someone, "Read me a story," then the traditionally appropriate response is to read from a storybook. It could be a book with lots of illustrations and just a few words each page (for very young children), or, conceivably, it can be something second graders read with as many as dozens of pages. But either way, it's generally for the young. I would never qualify Mirage TMNT comics or most mainstream adult fiction as "storybooks" in that sense, though something that condenses or licenses material from one of the cartoon series for young audiences might certainly qualify.
That said, you do bring up an interesting point: This term, though defined this way in mainstream dictionaries, may have variable meanings by regional culture that you or I may not be completely familiar with, and it may warrant at least a little bit of research and fact-checking. For example, wikipedia:storybook redirects to wikipedia:screenwriting for some reason, though we can't necessarily infer that redirect as something decided upon by a committee, since Wikipedia redirects are easy to make and just as easily overlooked. A deeper investigation into the redirect's edit history reveals that it previously had a very illogical redirect before being changed by one other editor apparently without any more formal review process, so I'm inclined to chalk that up to original research and insufficient oversight, both of which Wikipedia frowns upon.
Without a clearer guide for how the term can vary by region (if at all), I'm inclined to stick with the common dictionary definitions, and both North American and British dictionaries appear to share a common definition: books containing stories, especially for children. In its broadest definition it conceivably can mean any book that contains any story, no matter how adult and inappropriate for children, but at some point using the word in that sense stretches the definition too far for most people because of the "especially for children" qualifier. It would be like saying all adults are children by virtue of having been born to parents; a middle-aged man may be someone's child, but calling him a "child" in the broad sense would be weird and misleading. Words don't just have dictionary definitions—they also carry cultural connotations and fuzzy qualifiers that can't be completely ignored, which is why some dictionaries carry extra usage notes for each term that try to document these things.
I'm sorry, but there's no need of those Jesus and Mohammed pages, there's a moment when creating pages for the wikia when we kinda get a bit too far from the subject. I mean, those are just mentioned characters, they don't actually have a place of their own in the Ninja Turtles franchise. Just like we don't need to create pages for every city of every country the turtles have been, or monarchy pages, or federalism pages, etc.
The problem with Wikipedia is that it could've had almost everything (OK, not private persons, ordinary houses and so) but sadly the Deletionists (that's what they call themselves) at Wikipedia has set up "notability guidelines". This has caused a lot of dissapointed people to more and less give up Wikipedia (right now, I only contribute sporadical there).
Here, we write about things in a TMNT-perspective. I see no place for notability guidelines and such deletions here.