Not the most popular choice, but it was played recently by Abdusattorov in a rapid game. Bartel has also played it in November, so it was probably not a big surprise for Royal.
1. d2d4  1... d7d5  2. c2c4  2... c7c6  3. g1f3  3... g8f6  4. e2e3  4... g7g6 The Schlechter Variation is passive and leads to an advantage for White without any major problems or risks. But it's still a playable line, as will be demonstrated in this game. 5. b1c3  5... f8g7  6. f1e2  6... e8g8  7. e1g1 