When some older movies are shown in HD, if the OAR isn't widescreen than the HD presentation of it won't be...you'll get that little sidebar thing happening. I think the HD presentation of "The Wizard of Oz" was like this...
It is HD. The picture is the center 4:3 plus the sidebars and it is being transmitted at the same rate as full hd on that channel.
Now, is that 4:3 content HD. Most likely not. But it is being upconverted to HD.
16:9 does NOT have to be HD. 4:3 does not have to be SD. ASPECT ratio's are not tried to a standard, if you look at the ATSC standards. BUT, they usually are.