and it's not sufficient anyways. even with NX you could have the attacker control a function pointer you call. in any sufficiently complex program you'll have enough widgets to build anything you want just out of data.
code and data are the same thing. whatever inputs your system receives is "code" that your application interprets, just like your application is the code that the CPU interprets. turtles all the way down.
There is no fundamental distinction between code and data. This separation does not exist as category in actual reality (and I vividly remember that being one of the bigger points GEB made, early on). They're fundamentally one and the same, and classifying some bits as data and some as code is a point of view that really depends on what you're doing and why.
As for Harvard architecture - every time you branch on your "data", you're effectively executing it, and your "code" can be seen as emulating a system where your "data" is the actual code.