I'm just guessing, but I'd imagine I found something that's going to cause a debate or two.
See, we've already had canon statements from "Blade Squadron" of how even at the time of Endor, it was rare for someone to have seen a Star Destroyer. And even in A New Dawn, first of the novels of the new canon and set at least ten years earlier, a Star Destroyer is thought to be a new sight to an important Imperial mining system.
This brings us to the following scene from Chapter 17. Hera is checking up on a potentially interesting person that might be worth recruiting when she discovers he's a loony.
“The thorilide triangle,” Skelly said, astonished that she hadn’t heard about it. He moved across the room to the other side, with his wall of corporate shame. “The mining firms are corrupt. They’re tied up -- ownership, boards of directors -- with the shipwrights that have sold the Empire on one construction project after another. Oh, it’s all being done in secret, but you can’t keep everything secret. A billion Star Destroyers isn’t enough. They’re building Super Star Destroyers, and Super Super Star Destroyers, and who knows what else!”
“I see,” Hera said, gingerly taking a step backward. “And how do you know all this?”
“Oh,” Hera said. “The HoloNet.”
“It’s all one big web, and it goes on forever,” Skelly said, eyes fixing on the far wall. He stepped over to it and began fumbling with notes. “Did you know it was the moneyed interests that started the Clone Wars? There was a battle droid manufacturer that had too much inventory --”
Hera bugs out soon thereafter.
But, we do have the loony guy put "billion" and "Star Destroyers" in the same sentence, and I can just imagine folks in certain quarters declaring this as a valid ship count, perhaps also including Super Super Star Destroyers in their fleet list. They may even start scratching out what we know of the start of the Clone Wars and instead point toward "moneyed interests".
As for me, I'm not sure if he was just picking a wild number out of the air to make a point, or if he really believed there were a billion Star Destroyers because nobody can lie on the HoloNet, or if Palpatine had publicly ordered a billion-ship navy solely to cover up the Death Star's construction.
Given a million-system Empire, that would be 1000 ships each . . . and yet Endor was (see Insider 149 p. 56) "the largest flotilla of Star Destroyers ever assembled".
But, I can imagine that won't stop some people.