I think this is largely marketingspeak. And they stole it from engineers who were trying to convey that some EQ voicings are just more right than others when you're working with EQs that have limited controls.
In particular, when you've got an EQ that's not fully parametric, or configured with enough bands to cover the entire audible frequency spectrum, you're at the mercy of what some electrical engineer thought was the best parameters for Q and taper and whatnot on the EQ.
This is usually the case when you're working with channel strips on analog mixing consoles: each strip has an EQ section, but it's simple in what you can do because there's limited room on the physical strip for potentiometers to play with. As such, the channel strip designer has to make some tough decisions about how that EQ can be controlled and how each dial changes the EQ voicing. A "musical" EQ in this context would be one where, when you move a dial on the EQ, gives you pleasant results -- the Q and taper and center frequencies of the bands in the EQ all change in a way that makes working with the EQ easy.