Your assessment is correct in that any reasonably sized shell will protect the down just fine. There are two additional benefits beyond down’s more obvious benefits of 1) warmth, 2) weight, and 3) durability versus the Thermawrap's.
4) Down, worn as a mid layer, benefits from your much higher metabolic rate when doing camp chores versus sleeping. Also there is much more of a bellows effect when doing camp chores. Combined, these factors tend to continually dry out down insulation garments versus sleeping bags which will continue to accumulate insensible moisture unless hung to dry.
5) The outer shell has the added benefit of significantly improving the warmth of sewn through down garments. The shell provides an incremental ~12mm of insulating dead air space on top of the down garment’s concave sewn-through seam channels. The outer shell significantly increases the warmth of the sewn through down garment more than it would with a uniform thickness garment.
There are environments where the Thermawrap would be a better choice for an insulation layer. Some that come to mind are the rain forests of SE Alaska, water sports activities such as packrafting, etc.
The down insulating garment is not a viable substitute for a base layer and a relatively light insulating layer for actually backpacking. Down under pressure from the pack harness will totally collapse. Also perspiration, from over heating, can collapse the down.