Hard to imagine this happening, but 8 others survived, including the Dutch man's wife, as reported here: https://www.theborneopost.com/2019/07/14/sar-operation-mounted-to-locate-missing-guide-in-mulu-national-park/
Another report, with video, showing search area near upper entrance, at sink for Deer Water Cave https://www.newsarawaktribune.com.my/ops-sar-mulu-search-for-missing-guide-continues/
This is sure to be beyond the boardwalk show cave section of Deer cave. At the upstream end 'adventure' groups can descend from the end viewing platform down to the river (which they have to cross) and out into the Garden of Eden, an impressive mega doline. The river which enters the upstream end of the through cave does not follow it all the way. It sinks in a sump at a much lower level before reaching the show cave section itself. This water then emerges at another cave called Deerwater.
My guess is that the two got washed away on their return journey either crossing the river in flood or along a section where you scramble on terraces just above the river but where escape upwards is steep. Poor chaps.
The guide's body has now been recovered too - https://www.theborneopost.com/2019/07/15/body-of-missing-tourist-guide-found-in-mulus-deer-cave/
There is a reference in this report to "the newly discovered 75-metre passageway deep inside the cave" - from video in a previous report I suspect this may have opened up where the water sinks for Deer Water Cave...