Everything 'published' by GW (i.e. presented by GW as any sort of official GW thing) is canon.
However, canon=!official.
In terms of BFG Fluff, it is wisest to assume that an heirarchy exists with Rulebook>Armada>Online Rules*>Magazine>FOM (Fanatic Online Magazine)>Else, where lots of Else simply isn't 'official' at all.
* The 'short time only' Tau's Commerce Protection Fleet, the Adeptus Mechanicus etc.
Now, as I said, all GW publishes (i.e. something which is GW copyright and sanctioned to be published) 'is' canon. This doesn't mean the same as it does for, say, Star Wars. GW's policy is 'everything is canon, especially the contradictory bits'. So in that regard Squat fleets should be just around the corner.
The rule is a blanket rule which enables them not to enforce (or arguably waste money, time and effort) canon by stressing out over whether a ship is 3km or 3.5km long. One need only look at Dan Abnett. His works are often rather enjoyable reads and they certainly are amongst the BLP's best-sellers, but they often trundle over folks' ideas of canon like a steamroller over some flimsy origami birds.
By the same token, Codices and Rulebooks make mistakes all the time. There's a loose 'hymnsheet' to which the studio attempts to be singing off of, but let's not stress too much, as long as we're in the same ballpark. Or maybe even just on the same dreamscape...
