I checked out the submission requirements, and they merely ask for the list of your social media presence (that would be your website, twitter, facebook, tumblr, etc.).
I don't see this as a red flag at all. They're not requiring you to have an online footprint, they just want to know if you do. I've seen little publishers require that you have X amount of twitter followers/facebook likes, etc. (Which I think is dumb.)
If you don't have any footprint on the Internet, don't list anything. Much like if you have no previous published works, you list nothing in your query for published works.
There *is* a difference between marketing and promotion. Marketing is the stuff you don't see, behind the scenes--getting books into stores. Co-op. Ads. Sending out ARCs for reviews. Etc.
Promotion is the author interacting with the world, more or less. Author probably should think about how they want to interact with the world, even if it's just a static website.
But chances are, especially with YA, that a publisher will ask you to interact with the world. Probably via guest posts and blog tours and the like. It's what the *readers* expect, like it or not.
It doesn't mean you'll have to be on twitter all the time hawking your book (in fact, please don't.)