Mounting light to vinyl siding block?
Your guess is correct.  The mounting block shown in the Family Handyman image has a self contained wet rated box.

You cannot run Romex to your fixture and connect it behind the mounting block and expect it to pass inspection.  The mounting block in your image would be appropriate for attaching an outdoor electrical box on top, to house a receptacle or light switch for example.

Here's a mounting block containing a wet-rated box:

https://www.lowes.com/pd/Durabuilt-8-5-in-x-7-5-in-White-Woodgrain-Vinyl-Electrical-Mounting-Block/1000047117