If that's the case then the 2 sentences quoted above contradict themselves. Even if Greek and Roman cultures are similar to each other then they are both still different to Norse. Therefore their similarity is not relevant to why you would hope they didn't use other mythologies.
Personally I'd argue Greek, Roman and Norse mythologies are similar enough where yeah when you have one you have all 3. Not only that, Norse (IMHO) is pretty easily the neatest.
If they were going to explore other mythos than Norse then I would vote for something other than those 2 myself. Still, there's plenty of meat the Norse bone for multiple games methinks.
The problem with a lot of the mythologies is we simply don't have enough information to reasonably reconstruct them. Greco-Roman mythology (which was intermingled enough to basically be the same, just with Latin vs. Greek names for the gods) is reasonably well documented, just because it was taking place in the context of a literate society with extensive surviving records. Norse is a bit trickier, relying on a few sources and some sagas; we have plenty of references to events that we have no idea what they refer to (e.g. most of Loki's accusations in the Lokasenna).
It gets even worse for other mythologies; records are few if any (what did e.g. the Mississippian culture believe? we have only the faintest idea), and what research is available is often not easily accessible.
A lot of it is unreliable as well, for a variety of reasons. To take an example I'm somewhat familiar with, consider the Aztec god Quetzalcoatl. Most of our early sources about him come from either the reports of Spanish conquistadors (which have obvious issues), a few legal proceedings (e.g. native nobles petitioning for their right to certain land or inheritance based on familial histories, which likewise have issues), or from native reports collected by Spanish missionaries. The problem is that most of these are coming from a Christian context, and with their own biases. In particular with Quetzalcoatl, a belief developed (for various theological, historical and ideological reasons) among many of the early modern Spanish missionaries that he was actually a Christian (and often specifically the Apostle Thomas), whose memory had become corrupted among the natives over the succeeding years. As a result, early modern Spanish historians tended to emphasize things that played into this belief, and minimize things that might contradict it; in addition, they propagated it among Latin Americans themselves, such that even many natives (who, after all, hadn't worshiped Quetzalcoatl in generations by this point) believed it. As a result, by the 18th century, you have native descriptions of Quetzalcoatl as a bearded white man with a cassock and tonsure, preaching the gospel. Sifting the "original" belief (which may not have ever been completely codified) from later accretions is still somewhat difficult to this day, and that's in a situation where we have records dating back to the Conquest itself.
STOP! COOKIE TIME
