It's a bit of a mess regardless of if you take its ancient origin at face value or if you believe that it was made up from Heath's imagination, since the Scarlet book is 200 years old and the Brass Tower only burned down 150 years ago. So Heath and his team shouldn't really have been familiar with Suicune(or any of the other legendary beasts that inspired the sketch) in the first place. Unless there's 50+ years between the time GS and SV take place which I find pretty unlikely.
This is why I think its more a combination.
Here's my speculation though I'vs seen other people suggest similar
I think its a bootstrap paradox of a sort. I think the paradox Pokemon were unintentionally created by Turo/Sada (inspired by heath's books) but then were sent to heath's time where heath encountered them (either physically or hallucination/future vision etc) and then inspired their own creation when he wrote the books based on what he either saw or believed he saw. Thus heath and his team would not need to be familiar with the brass tower.