He can say he's a SF all he wants. What position he plays is going to be up to the organization that drafts him and his play on the field.
I haven't seen enough of Derrick Williams to make a real knowledgeable judgement on his ability. However, from what I did see did not lead me to believe he could be a full-time NBA SF right away. Maybe his handle is tighter and better than I'm thinking, I don't know.