Why are people still relying on outdated positional designations? Debating whether Johnson is a "SG" or "SF" is foolish because he's both/neither; he's a wing. There's no difference between those positions.
What he's not, is a big wing. Of course 6'6'' is fine for an "SF", but 216 with a 6'9.25'' wingspan isn't going to cut it against the likes of James, Durant, S bag, Antetokounmpo, etc.
That's why he doesn't fill a need, because they already had Murray, White and Walker who already are or project as average or better defenders from 1-3.