Signature Capture is a "RhoElements" features (http://docs.rhomobile.com/rhoelements/apicompatibility) and is listed on the matrix in the link provided. So, it does require a RhoElements license.
One question, have you added a license to the build.yml section? I'm just trying to see why you would be getting a "License Confirmed" pop up as well as a "Please supply RhoElements license key" nag screen.
No, there is no license given in the build.yml file.
I do get the warning at the end of the device build saying that a license is needed.
Yet I get the sequence of 3 popup messages. (Most of the time - inconsistent, as I noted above.)