It's literally not. If you have 10 windows open that's up to 9 presses of alt+tab. With workspaces it's one press of whatever you have configured for the workspace, such as alt+1 (or 2 or 3, etc)

True, switching to workspace allows you to omit some keypresses since you immediately get to a smaller bucket of windows.

But then once you get the workspace you want, more keypresses are to get it in focus.

The maximum number of operations will still tend to be higher especially if you use the keyboard. You have to switch to the correct workspace, the correct monitor, and then the correct window. If the workspace has 6 apps on it you realistically probably wont want to alt tab up to 5 times.

I would if I were still using say KDE hit a hotkey on the mouse to scale out, click a workspace, then hit a hotkey to display all windows on that workspace and select the correct one. This reduces it to 2 operations which isn't horrible but it still requires one to attend to the screen in between and pick up the mouse which makes the operations both more cumbersome and slower.

It's also very natural on i3 to have a lot of one application workspaces or one with 1-2 utility windows meaning most switching operations have exactly one stop something that you COULD do on KDE but people generally don't because the shape of the UI doesn't encourage it.