"Maybe" and "Possible" isn't definitive evidence of anything, and so far as I know, that book depository is the only source I know in which it says that "The long-awaited conclusion to the Hilda graphic novel series by Luke Pearson." Every other site I could find, doesn't say it, the Goodreads, Ebay, Amazon, Barnes & Noble, Google Express, and Kindle Bookstore, none of them say it is the long-awaited conclusion of the series. It's just one single source, which isn't convincing.
Besides, maybe, that phrase refers to the fact that it is the end of the Stone forest arch, and not the series as a whole or maybe it's just a misphrasing on their part, I'm sorry, but until Luke Pearson himself, goes on twitter or a panel and confirms that this the very last of the series, then I won't buy it(speaking of which wouldn't he have said something by now, if this was true, I mean if it was the very last one, don't you think he would've said something about it?).
And as for the publisher saying "possibly" it doesn't mean it WILL BE CANCELLED, it just implies that if Luke Pearson wanted to, it could be the last one, but it still isn't definitive of anything.
Again stop spreading rumors.