Who says the adventuring NPC wizard can make the scroll? That goes right back to what I said. Them simply being NPCs doesn't grant them that capability. Rather, they have that ability only if the DM deems it appropriate for them. And if your DM gives that ability to "you but an NPC," but doesn't let you have it, that's a DM problem, not a rules problem.
The point of NPCs not following PC rules is that they are not assumed to be adventurers. NPC Wizard Bob is not able to create 9th level scrolls because he is equivalent to a 17th level caster. No, NPC Wizard Bob can create 9th level scrolls because he retired from adventuring decades ago and has been honing his scroll creation ability ever since. He's just as useless as the PC when it comes to brewing complex potions or crafting magical hats, because those are not the NPCs area of expertise.
But yes, an NPC the DM creates that is a master magical craftsman will be... a master magical craftsman. And that is not a skill an adventurer can get by adventuring.