Yeah it's silly marketing
The job of rendering elements to the screen is the job of a layout manager. It's responsible for figuring out where elements are, if they are invalidated by changes, need to be painted etc. In response it will set about painting damaged / dirty elements in turn and then refresh the screen. A really fancy layout manager might recognize that some elements haven't changed between invocations and cache the result in a bitmap or surface to save having to individually ask each one to render from scratch.
But the layout manager is usually the last in a long chain of other things that must happen first. DOM manipulations, application of CSS, updates from JS & plugins, So claiming to hardware accelerates the lot is rather meaningless. What does acceleration even mean? Does it mean that they use a hardware blit to shift everything into the screen or accelerated primitives to render the content? What about blending & effects? What about 3D?
It's such a vague term that it's meaningless. I would hope every browser makes at least some use of hardware acceleration. What matters is how long it takes to render, not the means taken to get there.