Yeah, I just don't follow that logic at all. We know for a fact it is far future cosmere. Regardless of whether or not it has dragons, these will be things Brandon considered. Dragons would only be one more element and not inherently more spoilery than anything else. If it was a discussion on whether or not the book is in the future or high connectivity, I could see this being a point, but it's not; we know it is those things. He already chose to write the "harder" story with more to think about. Dragons may or may not be involved, but their presence does not add complications that Brandon hasn't already accepted (maybe even been enthusiastic about) when writing the book.
Sunlit Man
I don't think Brandon went "I want to write about dragons, let's make this harder for myself". I think he went "I want to write this thing, and there are dragons in it". It could be that all the dragon talk is unrelated and he simply has them on the mind! But if they are in the secret project, I don't think that complicates it any more than he already inherently chose to deal with.
He also has total control over what he writes. If he doesn't want to spoil something, he can find a million ways to do that. He can select the exact plot and characters he wants in order to release the information he wants to.