It's a flashback. The biggest tell is the appearance of Chrollo, which matches how the manga depicts him during the troupe's formative years.
Hypothesis: could this image be depicting the moments directly prior to the Kurta massacre? If Shalnark is Kurta, or connected to them, this could explain why he looks like he is running to stop them. Being that Uvo and Nobunaga have been established as the main frontline of the troupe, it would make since then why they are in front, if they are indeed about to start the massacre.
Another little thing I've noticed (let me know if its been noted before) - Shalnark's color scheme in the Nippon Animation is the same as Pairo's. Odd coincidence?