It is a blanket statement for all drives of all speeds.
A more in-depth explanation is to do no lower than half the rated speed of your media, and do no higher than 1 speed lower than rated speed.
Or, depending on what speeds your drive offers, try to do no less than half the rated speed of your media. If not possible, do no less than half the speed offered for your media.
So if you have 16x media, stay between 8x and 12x [on most drives rated at 16x or above].
If you have 8x media, stay at 6x or 8x on most drives [assuming that your drive is rated at 8x or above].
If your media is 16x rated, but your drive can only do 8x max [on your media], try to stick with 6x or 8x, depending on what works for you.
If you’re using an older drive that has no support for your media whatsoever, or is a slower-speed drive, and leaves you with a low speed like 4x for 16x media, do 4x.
And so on and so forth.
Just don’t expect your drive to do as well as a drive that can support a media’s maximum speeds.
And for drives that overspeed media: overspeeding is not recommended. So with a 22x drive, you’ll still want to try to stay around 12x max for 16x media. And so on and so forth.