If cargo anchors are meant to be used as tether anchors, the owner's manual should indicate that, and I would start there first, with calling the manufacturer being step 2 if need be for further clarification.
Cargo anchors and tether anchors are only synonymous if the manual or manufacturer says they are, and it can totally vary from vehicle model to vehicle model. Cargo anchors have no FMVSS, unlike tether anchors, and some cargo anchors aren't designed to bear much other than smaller items in a lightweight cargo net, for example. I'm not sure what black bars you're talking about in the back of your Sienna.
The actual tether anchors are on the lower back of all of the rear seats except for the third row driver's side in the '04 and newer Siennas. There are silver brackets below the Sienna's rear cargo sill and those silver brackets are intended to hold an optional cargo net, not to serve as tether anchors.