I have not done the fine testing to see if there are short bursts of export. I'm sure there is, as if you think about it - any system that measures and then responds is going to have some finite amount of lag. Unless you maintain a large buffer of input power from the grid as Victron does, it will be impossible to guarantee no export at all.
For me - I plan using this on a subpanel, so if a little gets pushed backwards into the rest of my house, it is not a big deal. I'm essentially maintaining a buffer of input power by virtue of using this on a subpanel.
Long term - I don't want ANY export either. If I were to use this on my main panel, I would probably set up a small load - perhaps 100-500 watts that are in place up-stream from the CT's. That would be my buffer. I would probably also use a current activated switch to shut down the grid connection to the NHX if any reverse flow was detected.
The nice thing about this inverter - for me, is that it is big enough that I can power a lot of loads on the output, and I don't HAVE to use the export feature if I don't want to. It's appealing to use it, because I don't have to do a lot of rewiring and shifting things around.