See attached screenshots and reports from chrome://gpu/

When the --disable-gpu option is passed to the web engine, chrome://gpu/ reports that all hardware acceleration is disabled. However, when loading and scrolling a webpage,

• Google Chrome 98.0.4758.82 truly does not use any GPU
• Qt WebEngine still uses the GPU (tested with the Simple Browser example) 

Notice that Qt WebEngine's copy of the report still contains --create-default-gl-context --use-gl=desktop  --in-process-gpu --gpu-preferences=... 

Partial workaround

Setting the environment variable QT_QUICK_BACKEND=software will properly disable the GPU. However, this requires sacrificing hardware acceleration in all Qt Quick code.