The flag to control hardware encoding is "--disable-cast-streaming-hw-encoding."  But the description suggests the flag actually enables hardware encoding:

"This option enables support in Cast Streaming for encoding video streams "
"using platform hardware.";

https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.cc?rcl=c244be81d62b853a06d41a330f94a8273e128628&l=158

Or, is the flag still needed at all?