Semicolon with "namely" and "for example"

I have been looking through the punctuation section in The Chicago Manual Of Style and have read something that does not make sense to me.

It states that you should use either a semicolon or an em dash before the expressions "namely", "that is", and "for example". From the examples that they give, I am confused about how a semicolon would be an acceptable choice.

Here is one of the examples:

"There are simple alternatives to the stigmatized plastic shopping bag—namely, reusable cloth bags and foldable carts."

True, the example uses an em dash, but they suggest a semicolon could also work. Wouldn't this be incorrect as the phrase after "namely" is not a standalone sentence?

It is section 6.43 if anyone has the CMOS (16th edition) to hand and wants to look it up. It is possible I have just misunderstood.