It looks like (according to the video) that you can set the witness option to on or off. I would be very surprised if any agency accepted a release without a witness.
I'm a big fan of getting things out of paper and onto a device but I'm still very nervous about not having a hard copy of a release. I can see how where banks, post office, credit card machines etc. all use a digitally signed signature as legal tender that this should be good too, but when push comes to shove in a legal court - having a hard paper copy with real pen ink on it would make me feel better.... whether one is more 'legal' or 'binding' than another .. I don't know.
That said, having the app in case you run out of releases, or forget them at home or something dumb, isn't a terrible idea. It is nice to see that iStock accepts the release as well.