Can't point you to a history source, but, no, gravel bikes are a type of road bike, meaning drop bar bikes originally used for pavement riding, not a mix of hybrid bikes and mtbs, both of which typically use flat bars of one type or another. The key difference is drop bars versus various types of flat bars. In other words, if you walk into a bike shop and ask to look at gravel bikes, you'll be shown bikes with drop bars. If you ask to see hybrids or mtbs, you'll be shown bikes with flat bars. You can ride gravel roads on any kind bike, of course. Gravel bike in today's lingo means drop bar bike, though.
The gravel bike movement started when some adventurous road bikers decided to venture off into gravel riding. That opened up many thousands of roads to ride and the design to make riding gravel more doable followed. Today, gravel bikes are basically road bikes with endurance (more relaxed) geometry than performance road bikes and clearance for larger tires for the sake of riding dirt and gravel.
You can certainly use gravel bikes or any kind of drop bar bikes for commuting, but the key to whether your wife will be happy with a gravel bike will mostly depend on how comfortable she is riding drop bar bikes. That's can be quite an adjustment if she's new to drop bars. There's a learning curve and time period needed to get comfy with riding a drop bar bike.