From the article it seems like this ROV doesn't have sonar, and can only search visually at a short distance. They claim a distance of around 20 feet. I'm surprised they could find a debris field so fast with such a small range of observation.
1) hydrophones heard it implode at the same time it lost contact
2) it took until early 6/22 to get a deep-diving ROV on site
3) once the ROV got to the bottom, it swiftly found the debris from the implosion as expected
EDIT: Looks like the US Navy heard it happen, as per the WSJ:
https://www.wsj.com/articles/u-s-navy-detected-titan-sub-imp...
EDIT2: HN discussion: https://news.ycombinator.com/item?id=36439661