What build system are you using? Ninja is multithreaded by default so it will be ignored but systems like msbuild, Make, xcodebuild, iirc are single threaded by default and you can promote them to multi threaded by using -j or just passing their multithreaded option like

cmake --build <binarydir> --config Debug -- /m # msbuild

if cmake is actually ignoring the -j flag. If it is you should create a ticket on cmakes gitlab repo