“(Not) grabbing the keyboard” doesn’t mean you don’t get keyboard input at all, it’s about whether keyboard input can also be handled by the desktop environment/window manager.
If your SDL app does grab the keyboard, global keyboard shortcuts/hotkeys won’t work, so you can’t switch to another applications with ALT-TAB or open the start menu with the windows key or turn up/down the speaker volume with volume keys etc.
If it does not grab it (which is the default), these global shortcuts will work, and you’ll still get keyboard events for everything that is not handled by whatever handles those global shortcuts.
Hope that clears it up
If you want to disable keyboard events, just set some global flag and, wherever you call SDL_PollEvent(), ignore/skip keyboard events if the flag is set.