Dengeki have issued a platitudinous apology for their use of illustrations traced from deceased artist 堀部秀郎 / Hiderou Horibe’s works; they promise it won’t happen again (it has in fact happened several times previously, and they were most evasive in this case until exposed thoroughly on 2ch).
What remains unexplained is the identity of the elusive 桜本ナオヒサ / Sakuramoto Naohisa; nobody has ever heard of him, he has no online presence, and we hear no kowtowing from him.
Either he is a truly incompetent pirate trace artist, and so justifiably unknown (though how he would land such a job is a mystery), or “he” is a convenient name behind which to do some copying. Notably, Dengeki already has a reputation for this sort of thing.
Interestingly, the first sentence of their apology lays the blame firmly on Sakuramoto… “Illustrator Sakuramoto Naohisa drew works which formed part of our illustrations, and were copied in part from Horibe’s works”.