Most likely they're interpreting vessels operating in disputed 9dash eez as DWF instead of domestic fleet. So they magically discovered 12,490 new ships not claimed by China, which would not consider them DWF vessels. See table on page 15 of report.
> 12,490 vessels without IMO or RFMO registrations but with active AIS signals outside Chinese waters at some point between 1 January 2017 and 31 December 2018.
AND
>AIS signal outside Chinese EEZ in 2017 or 2018
CTRL+F entire report "South China Sea", 1 hit from references. The fact that this point doesn't get any elaboration makes this report suspect as hell. That leaves 4,476 vessels, 927 foreign flagged (20.7%) consistent with Chinese numbers. Under 4.3.1 (pg 21): "around 20% of the world’s fishing vessels are registered in states to which they have no other connection". In that same paragraph it tries to use 927/16,966 to show Chinese share of foreign vessels is relatively small at 5.5% when using 4,447 its right at the 20% mark.
Also shifty eyes at 2.2.1 that tries to do a nominal comparison to EU/US DWF fleets but conveniently leaves out the other countries with the largested DWF fleet size (Taiwan, SK, Japan, Spain) [1]. Taiwan adjusted per capita would have a DWF fleet the size of 24k ships. S.Korea is around 5K ships which is in line with China, per capita, but it seems like relative numbers lke per capita is frowned upon if it doesn't make China look bad relative to regional peers.
https://www.odi.org/sites/odi.org.uk/files/resource-document...
[1] https://www.stimson.org/wp-content/files/file-attachments/St...