I have important css in the plugin now.
To reduce bloat and plugins, Kinsta recommended instead of Simple CSS :
(use the WordPress Customizer or GP premium hooks to load custom CSS only where you need it)
Does he mean the “add css” area on custumizer?
there is no difference between using Simple CSS and the Customizer > Additional CSS. Both of them output their CSS inline to the head of the site. The advantage of Simple CSS is if you only require it on one page you can add it to the Simple CSS meta box.